oauth2: add support for client credential grant type

Creates a new package called clientcredentials and
adds transport and token information to the internal
package. Also modifies the oauth2 package to make
use of the newly added files in the internal package.

The clientcredentials package allows for token requests
using a "client credentials" grant type.

Fixes https://github.com/golang/oauth2/issues/7

Change-Id: Iec649d1029870c27a2d1023baa9d52db42ff45e8
Reviewed-on: https://go-review.googlesource.com/2983
Reviewed-by: Burcu Dogan <jbd@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

internal/token_test.go

@@ -0,0 +1,28 @@
+// Copyright 2014 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 internal contains support packages for oauth2 package.
+package internal
+
+import (
+	"fmt"
+	"testing"
+)
+
+func Test_providerAuthHeaderWorks(t *testing.T) {
+	for _, p := range brokenAuthHeaderProviders {
+		if providerAuthHeaderWorks(p) {
+			t.Errorf("URL: %s not found in list", p)
+		}
+		p := fmt.Sprintf("%ssomesuffix", p)
+		if providerAuthHeaderWorks(p) {
+			t.Errorf("URL: %s not found in list", p)
+		}
+	}
+	p := "https://api.not-in-the-list-example.com/"
+	if !providerAuthHeaderWorks(p) {
+		t.Errorf("URL: %s found in list", p)
+	}
+
+}