Implements group_by, projection and offset on datastore Query.
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.
Makes operator checking deterministic in Query.filter.

The order of dictionary keys is not guaranteed between runs of
code so `expression`s ending in '>=' or '<=' were sometimes
matching for '='.
Adding test to make sure the correct operators get set on filter.
Only using explicit namespace when deserializing key pb.

This issue surfaced during review for #282.
Adding checks for namespace in datastore helpers test.

This is to ensure that values are correctly ignored when
the field isn't set on the protobuf.
Adding support for queries in regression test.

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