Google App Engine ndb async support

- Added channel for Google App Engine NDB, non blocking async IO. Uses *_async ndb convention, returning a future.
- Added documentation on how to override json serialization, to lessen the need for forks for custom json.

I have run basic tests locally as part of bigger system with GAE SDK test case setup and mock (as opposed to mox), but did not mock the GAE urlfetch with mox tests.