oauth2: new endpoints

Added *.Endpoint constants for facebook, linkedin, odnoklassniki, paypal
and vk.

Related to issue #41.

Change-Id: Ib447ff773a540aa9bc932a0e18d9bb9ef8703f87
Reviewed-on: https://go-review.googlesource.com/7370
Reviewed-by: Burcu Dogan <jbd@google.com>

facebook/facebook.go

@@ -0,0 +1,16 @@
+// Copyright 2015 The oauth2 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 facebook provides constants for using OAuth2 to access Facebook.
+package facebook // import "golang.org/x/oauth2/facebook"
+
+import (
+	"golang.org/x/oauth2"
+)
+
+// Endpoint is Facebook's OAuth 2.0 endpoint.
+var Endpoint = oauth2.Endpoint{
+	AuthURL:  "https://www.facebook.com/dialog/oauth",
+	TokenURL: "https://graph.facebook.com/oauth/access_token",
+}