Add User service.

service/user/interface.go

Index: service/user/interface.go
diff --git a/service/user/interface.go b/service/user/interface.go
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+++ b/service/user/interface.go
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+// Copyright 2015 The Chromium 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 user
+
+// Interface provides access to the "appengine/users" API methods.
+type Interface interface {
+	Current() *User
+	CurrentOAuth() (*User, error)
+
+	IsAdmin() bool
+
+	LoginURL(dest string) (string, error)
+	LoginURLFederated(dest, identity string) (string, error)
+	LogoutURL(dest string) (string, error)
+
+	OAuthConsumerKey() (string, error)
+}