App Engine: implement IndexStorage.Get

Change-Id: I7f123df494f32f5f8a28bbd8e4d0aeb11aecce8b
Fix JavaScript UI things I broke earlier when simplifying signing discovery

Change-Id: Idf4effe3cbf97ec4ccb51197a0b3c857cb7c3569
More App Engine index work.

But fail: 2013/01/09 03:38:54 replica: receiving blob, 1 successes, 1 failures; last error = API error 1 (datastore_v3: BAD_REQUEST): ApplicationError: 1 operating on too many entity groups in a single transactio

See http://stackoverflow.com/questions/8251594/how-many-objects-is-too-many-for-in-a-single-transaction-to-googles-datastore

Sigh.

Change-Id: I90dff1b6bd40a00b4be19c5599583d2131b739d4