Add simplest service/urlfetch implementation.

impl/prod/urlfetch.go

+// 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 prod
+
+import (
+        "net/http"
+
+        uf "github.com/luci/gae/service/urlfetch"
+        "golang.org/x/net/context"
+        "google.golang.org/appengine/urlfetch"
+)
+
+// useURLFetch adds a http.RoundTripper implementation to the context.
+func useURLFetch(c context.Context) context.Context {
+        return uf.SetFactory(c, func(ci context.Context) http.RoundTripper {
+                return &urlfetch.Transport{Context: ci}
+        })
+}