add GDrive provider support (#118)

* GDrive provider support
* More reliable basedir ownership
* Fix mimetype

vendor/golang.org/x/oauth2/google/appengineflex_hook.go

@@ -0,0 +1,11 @@
+// Copyright 2015 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 appenginevm
+
+package google
+
+func init() {
+	appengineFlex = true // Flex doesn't support appengine.AccessToken; depend on metadata server.
+}