Add key for funtional testing
Implements group_by, projection and offset on datastore Query.

In addition, __eq__ was implemented on datastore.key.Key
to allow for easy comparison.
Addressing corner cases for datastore.Key.__eq__.

These corners are a direct result of #274.
Remove notes about datetime.date objects and timezone matching.

Issues #278 and #279 have been filed to address.
Document regression tests, make run_regression fail if environ not set.
Adding support for queries in regression test.

This is an attempt port the gcloud-node regression tests
for queries over to gcloud-python.