heroku: Add Heroku's endpoints

The endpoints are documented here:
https://devcenter.heroku.com/articles/oauth

Change-Id: Ie8012d7588fc29afda7dd64f999c7418c32a5df2
Reviewed-on: https://go-review.googlesource.com/27635
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

heroku/heroku.go

@@ -0,0 +1,16 @@
+// Copyright 2016 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 heroku provides constants for using OAuth2 to access Heroku.
+package heroku // import "golang.org/x/oauth2/heroku"
+
+import (
+	"golang.org/x/oauth2"
+)
+
+// Endpoint is Heroku's OAuth 2.0 endpoint.
+var Endpoint = oauth2.Endpoint{
+	AuthURL:  "https://id.heroku.com/oauth/authorize",
+	TokenURL: "https://id.heroku.com/oauth/token",
+}