New aestubs API for NewContext, as well as new default stubs.

NewContext now is more usefull, it returns the full set of stubs
by default instead of requiring one to setup them manually.

Basic taskqueue and urlfetch services are also implemented now,
allowing basic assertions for enqueued tasks as well as basic
URLFetch functionality.
Updates documentation to new aestubs API.
Initial implementatio of the datastorev3.Run_Query method