Add cloud datastore implementation.

impl/cloud/context.go

+// Copyright 2016 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package cloud
+
+import (
+        "google.golang.org/cloud/datastore"
+
+        "golang.org/x/net/context"
+)
+
+// Use installs the cloud services implementation into the supplied Context.
+//
+// This includes:
+//      - github.com/luci/gae/service/info
+//      - github.com/luci/gae/service/datastore
+//
+// This is built around the ability to use cloud datastore.
+func Use(c context.Context, client *datastore.Client) context.Context {
+        cds := cloudDatastore{
+                client: client,
+        }
+
+        return cds.use(useInfo(c))
+}