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.
Merge pull request #292 from dhermes/unmarshall-key-namespace

Only using explicit namespace when deserializing key pb.
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.
Adding support for queries in regression test.

This ports the gcloud-node regression tests for queries
over to gcloud-python.
Making tox regression handle all local and Travis simultaneously.