Add tests for ndb_persistence

Add unit tests for ndb_persistence module
Add apache header to test_ndb_persistence
Switch to batch method that doesn't require a filter

Instead of using izip_longest instead use a slice batch method so we can avoid
an extra filter of None values.
Merge branch 'context-completion-persist-results' into completion_integration

Conflicts:
	furious/extras/appengine/ndb_persistence.py