diff --git a/vendor/golang.org/x/oauth2/internal/client_appengine.go b/vendor/golang.org/x/oauth2/internal/client_appengine.go
new file mode 100644
index 0000000..7434871
--- /dev/null
+++ b/vendor/golang.org/x/oauth2/internal/client_appengine.go
@@ -0,0 +1,13 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build appengine
+
+package internal
+
+import "google.golang.org/appengine/urlfetch"
+
+func init() {
+	appengineClientHook = urlfetch.Client
+}
diff --git a/vendor/golang.org/x/oauth2/internal/doc.go b/vendor/golang.org/x/oauth2/internal/doc.go
new file mode 100644
index 0000000..03265e8
--- /dev/null
+++ b/vendor/golang.org/x/oauth2/internal/doc.go
@@ -0,0 +1,6 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package internal contains support packages for oauth2 package.
+package internal
diff --git a/vendor/golang.org/x/oauth2/internal/oauth2.go b/vendor/golang.org/x/oauth2/internal/oauth2.go
new file mode 100644
index 0000000..c0ab196
--- /dev/null
+++ b/vendor/golang.org/x/oauth2/internal/oauth2.go
@@ -0,0 +1,37 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package internal
+
+import (
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
+	"errors"
+	"fmt"
+)
+
+// ParseKey converts the binary contents of a private key file
+// to an *rsa.PrivateKey. It detects whether the private key is in a
+// PEM container or not. If so, it extracts the the private key
+// from PEM container before conversion. It only supports PEM
+// containers with no passphrase.
+func ParseKey(key []byte) (*rsa.PrivateKey, error) {
+	block, _ := pem.Decode(key)
+	if block != nil {
+		key = block.Bytes
+	}
+	parsedKey, err := x509.ParsePKCS8PrivateKey(key)
+	if err != nil {
+		parsedKey, err = x509.ParsePKCS1PrivateKey(key)
+		if err != nil {
+			return nil, fmt.Errorf("private key should be a PEM or plain PKCS1 or PKCS8; parse error: %v", err)
+		}
+	}
+	parsed, ok := parsedKey.(*rsa.PrivateKey)
+	if !ok {
+		return nil, errors.New("private key is invalid")
+	}
+	return parsed, nil
+}
diff --git a/vendor/golang.org/x/oauth2/internal/token.go b/vendor/golang.org/x/oauth2/internal/token.go
new file mode 100644
index 0000000..355c386
--- /dev/null
+++ b/vendor/golang.org/x/oauth2/internal/token.go
@@ -0,0 +1,294 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package internal
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"math"
+	"mime"
+	"net/http"
+	"net/url"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+
+	"golang.org/x/net/context/ctxhttp"
+)
+
+// Token represents the credentials used to authorize
+// the requests to access protected resources on the OAuth 2.0
+// provider's backend.
+//
+// This type is a mirror of oauth2.Token and exists to break
+// an otherwise-circular dependency. Other internal packages
+// should convert this Token into an oauth2.Token before use.
+type Token struct {
+	// AccessToken is the token that authorizes and authenticates
+	// the requests.
+	AccessToken string
+
+	// TokenType is the type of token.
+	// The Type method returns either this or "Bearer", the default.
+	TokenType string
+
+	// RefreshToken is a token that's used by the application
+	// (as opposed to the user) to refresh the access token
+	// if it expires.
+	RefreshToken string
+
+	// Expiry is the optional expiration time of the access token.
+	//
+	// If zero, TokenSource implementations will reuse the same
+	// token forever and RefreshToken or equivalent
+	// mechanisms for that TokenSource will not be used.
+	Expiry time.Time
+
+	// Raw optionally contains extra metadata from the server
+	// when updating a token.
+	Raw interface{}
+}
+
+// tokenJSON is the struct representing the HTTP response from OAuth2
+// providers returning a token in JSON form.
+type tokenJSON struct {
+	AccessToken  string         `json:"access_token"`
+	TokenType    string         `json:"token_type"`
+	RefreshToken string         `json:"refresh_token"`
+	ExpiresIn    expirationTime `json:"expires_in"` // at least PayPal returns string, while most return number
+}
+
+func (e *tokenJSON) expiry() (t time.Time) {
+	if v := e.ExpiresIn; v != 0 {
+		return time.Now().Add(time.Duration(v) * time.Second)
+	}
+	return
+}
+
+type expirationTime int32
+
+func (e *expirationTime) UnmarshalJSON(b []byte) error {
+	if len(b) == 0 || string(b) == "null" {
+		return nil
+	}
+	var n json.Number
+	err := json.Unmarshal(b, &n)
+	if err != nil {
+		return err
+	}
+	i, err := n.Int64()
+	if err != nil {
+		return err
+	}
+	if i > math.MaxInt32 {
+		i = math.MaxInt32
+	}
+	*e = expirationTime(i)
+	return nil
+}
+
+// RegisterBrokenAuthHeaderProvider previously did something. It is now a no-op.
+//
+// Deprecated: this function no longer does anything. Caller code that
+// wants to avoid potential extra HTTP requests made during
+// auto-probing of the provider's auth style should set
+// Endpoint.AuthStyle.
+func RegisterBrokenAuthHeaderProvider(tokenURL string) {}
+
+// AuthStyle is a copy of the golang.org/x/oauth2 package's AuthStyle type.
+type AuthStyle int
+
+const (
+	AuthStyleUnknown  AuthStyle = 0
+	AuthStyleInParams AuthStyle = 1
+	AuthStyleInHeader AuthStyle = 2
+)
+
+// authStyleCache is the set of tokenURLs we've successfully used via
+// RetrieveToken and which style auth we ended up using.
+// It's called a cache, but it doesn't (yet?) shrink. It's expected that
+// the set of OAuth2 servers a program contacts over time is fixed and
+// small.
+var authStyleCache struct {
+	sync.Mutex
+	m map[string]AuthStyle // keyed by tokenURL
+}
+
+// ResetAuthCache resets the global authentication style cache used
+// for AuthStyleUnknown token requests.
+func ResetAuthCache() {
+	authStyleCache.Lock()
+	defer authStyleCache.Unlock()
+	authStyleCache.m = nil
+}
+
+// lookupAuthStyle reports which auth style we last used with tokenURL
+// when calling RetrieveToken and whether we have ever done so.
+func lookupAuthStyle(tokenURL string) (style AuthStyle, ok bool) {
+	authStyleCache.Lock()
+	defer authStyleCache.Unlock()
+	style, ok = authStyleCache.m[tokenURL]
+	return
+}
+
+// setAuthStyle adds an entry to authStyleCache, documented above.
+func setAuthStyle(tokenURL string, v AuthStyle) {
+	authStyleCache.Lock()
+	defer authStyleCache.Unlock()
+	if authStyleCache.m == nil {
+		authStyleCache.m = make(map[string]AuthStyle)
+	}
+	authStyleCache.m[tokenURL] = v
+}
+
+// newTokenRequest returns a new *http.Request to retrieve a new token
+// from tokenURL using the provided clientID, clientSecret, and POST
+// body parameters.
+//
+// inParams is whether the clientID & clientSecret should be encoded
+// as the POST body. An 'inParams' value of true means to send it in
+// the POST body (along with any values in v); false means to send it
+// in the Authorization header.
+func newTokenRequest(tokenURL, clientID, clientSecret string, v url.Values, authStyle AuthStyle) (*http.Request, error) {
+	if authStyle == AuthStyleInParams {
+		v = cloneURLValues(v)
+		if clientID != "" {
+			v.Set("client_id", clientID)
+		}
+		if clientSecret != "" {
+			v.Set("client_secret", clientSecret)
+		}
+	}
+	req, err := http.NewRequest("POST", tokenURL, strings.NewReader(v.Encode()))
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	if authStyle == AuthStyleInHeader {
+		req.SetBasicAuth(url.QueryEscape(clientID), url.QueryEscape(clientSecret))
+	}
+	return req, nil
+}
+
+func cloneURLValues(v url.Values) url.Values {
+	v2 := make(url.Values, len(v))
+	for k, vv := range v {
+		v2[k] = append([]string(nil), vv...)
+	}
+	return v2
+}
+
+func RetrieveToken(ctx context.Context, clientID, clientSecret, tokenURL string, v url.Values, authStyle AuthStyle) (*Token, error) {
+	needsAuthStyleProbe := authStyle == 0
+	if needsAuthStyleProbe {
+		if style, ok := lookupAuthStyle(tokenURL); ok {
+			authStyle = style
+			needsAuthStyleProbe = false
+		} else {
+			authStyle = AuthStyleInHeader // the first way we'll try
+		}
+	}
+	req, err := newTokenRequest(tokenURL, clientID, clientSecret, v, authStyle)
+	if err != nil {
+		return nil, err
+	}
+	token, err := doTokenRoundTrip(ctx, req)
+	if err != nil && needsAuthStyleProbe {
+		// If we get an error, assume the server wants the
+		// clientID & clientSecret in a different form.
+		// See https://code.google.com/p/goauth2/issues/detail?id=31 for background.
+		// In summary:
+		// - Reddit only accepts client secret in the Authorization header
+		// - Dropbox accepts either it in URL param or Auth header, but not both.
+		// - Google only accepts URL param (not spec compliant?), not Auth header
+		// - Stripe only accepts client secret in Auth header with Bearer method, not Basic
+		//
+		// We used to maintain a big table in this code of all the sites and which way
+		// they went, but maintaining it didn't scale & got annoying.
+		// So just try both ways.
+		authStyle = AuthStyleInParams // the second way we'll try
+		req, _ = newTokenRequest(tokenURL, clientID, clientSecret, v, authStyle)
+		token, err = doTokenRoundTrip(ctx, req)
+	}
+	if needsAuthStyleProbe && err == nil {
+		setAuthStyle(tokenURL, authStyle)
+	}
+	// Don't overwrite `RefreshToken` with an empty value
+	// if this was a token refreshing request.
+	if token != nil && token.RefreshToken == "" {
+		token.RefreshToken = v.Get("refresh_token")
+	}
+	return token, err
+}
+
+func doTokenRoundTrip(ctx context.Context, req *http.Request) (*Token, error) {
+	r, err := ctxhttp.Do(ctx, ContextClient(ctx), req)
+	if err != nil {
+		return nil, err
+	}
+	body, err := ioutil.ReadAll(io.LimitReader(r.Body, 1<<20))
+	r.Body.Close()
+	if err != nil {
+		return nil, fmt.Errorf("oauth2: cannot fetch token: %v", err)
+	}
+	if code := r.StatusCode; code < 200 || code > 299 {
+		return nil, &RetrieveError{
+			Response: r,
+			Body:     body,
+		}
+	}
+
+	var token *Token
+	content, _, _ := mime.ParseMediaType(r.Header.Get("Content-Type"))
+	switch content {
+	case "application/x-www-form-urlencoded", "text/plain":
+		vals, err := url.ParseQuery(string(body))
+		if err != nil {
+			return nil, err
+		}
+		token = &Token{
+			AccessToken:  vals.Get("access_token"),
+			TokenType:    vals.Get("token_type"),
+			RefreshToken: vals.Get("refresh_token"),
+			Raw:          vals,
+		}
+		e := vals.Get("expires_in")
+		expires, _ := strconv.Atoi(e)
+		if expires != 0 {
+			token.Expiry = time.Now().Add(time.Duration(expires) * time.Second)
+		}
+	default:
+		var tj tokenJSON
+		if err = json.Unmarshal(body, &tj); err != nil {
+			return nil, err
+		}
+		token = &Token{
+			AccessToken:  tj.AccessToken,
+			TokenType:    tj.TokenType,
+			RefreshToken: tj.RefreshToken,
+			Expiry:       tj.expiry(),
+			Raw:          make(map[string]interface{}),
+		}
+		json.Unmarshal(body, &token.Raw) // no error checks for optional fields
+	}
+	if token.AccessToken == "" {
+		return nil, errors.New("oauth2: server response missing access_token")
+	}
+	return token, nil
+}
+
+type RetrieveError struct {
+	Response *http.Response
+	Body     []byte
+}
+
+func (r *RetrieveError) Error() string {
+	return fmt.Sprintf("oauth2: cannot fetch token: %v\nResponse: %s", r.Response.Status, r.Body)
+}
diff --git a/vendor/golang.org/x/oauth2/internal/transport.go b/vendor/golang.org/x/oauth2/internal/transport.go
new file mode 100644
index 0000000..572074a
--- /dev/null
+++ b/vendor/golang.org/x/oauth2/internal/transport.go
@@ -0,0 +1,33 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package internal
+
+import (
+	"context"
+	"net/http"
+)
+
+// HTTPClient is the context key to use with golang.org/x/net/context's
+// WithValue function to associate an *http.Client value with a context.
+var HTTPClient ContextKey
+
+// ContextKey is just an empty struct. It exists so HTTPClient can be
+// an immutable public variable with a unique type. It's immutable
+// because nobody else can create a ContextKey, being unexported.
+type ContextKey struct{}
+
+var appengineClientHook func(context.Context) *http.Client
+
+func ContextClient(ctx context.Context) *http.Client {
+	if ctx != nil {
+		if hc, ok := ctx.Value(HTTPClient).(*http.Client); ok {
+			return hc
+		}
+	}
+	if appengineClientHook != nil {
+		return appengineClientHook(ctx)
+	}
+	return http.DefaultClient
+}