vendor: add github.com/diegomarangoni/gcscache

also include all transitive dependencies of gcscache, which seems to
bring in the entire world of Google Cloud Platform.

vendor/golang.org/x/oauth2/client_appengine.go

@@ -0,0 +1,25 @@
+// 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.
+
+// +build appengine
+
+// App Engine hooks.
+
+package oauth2
+
+import (
+	"net/http"
+
+	"golang.org/x/net/context"
+	"golang.org/x/oauth2/internal"
+	"google.golang.org/appengine/urlfetch"
+)
+
+func init() {
+	internal.RegisterContextClientFunc(contextClientAppEngine)
+}
+
+func contextClientAppEngine(ctx context.Context) (*http.Client, error) {
+	return urlfetch.Client(ctx), nil
+}