New version with use of BaseDatastore and DatastoreStub.

This is much simpler, but brings some overhead.
Now supported querying.

Still some tests fail - ancestor queries etc.
Deleted wrong info and some garbage methods.
Edited README, deleted wrong notes.
Fixed offset error and anecstor query behaviour.
Fixed filtering by non-trivial property types.

Filter on types like datetime or geopt caused error. Added a few tests.
Support native mongodb indexes.
Updated README
Fixed dev_appserver and mongo stub to accept root_path param.

This was needed because of index treating in devel console.
Fixed XSRF token generation error in SDK devel console.

The error was caused by wrong manipulation with entity key id vs name.
Updated info in README.
Load saved indexes when the stub inits.
Datastore key is stored as array to improve ancestor query lookup.
Optimized _GetEntitiesInEntityGroup method for higher performance.

Introduced caching, deleted some of unused methods.