oauth2: rewrite google package, fix the broken build

Change-Id: I2753a88d7be483bdbc0cac09a1beccc4806ea4bc
Reviewed-on: https://go-review.googlesource.com/1361
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

internal/oauth2.go

@@ -2,6 +2,7 @@
 // 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
 
 import (