Adding namespace to dataset class.

[dhermes]

This does not add support for namespace in _implicit_environ.

Related to #902.

---

As a result of #861 we will likely be removing _implicit_environ which makes this the actual resolution of #902.

/cc @jonparrott

[dhermes]

FWIW I checked where namespace is used in the .proto API. It is only used in PartitionId (along with dataset_id) and the PartitionId message type is only used in Key and RunQueryRequest.

Accordingly, the only places we support namespace is:

• Connection.run_query(dataset_id, query_pb, namespace=...)
• Key(..., namespace=...)
• Query(..., namespace=...)

[theacodes]

Cool, as long as it's an option on the client class when that happens I'm satisfied.

[tseaver]

LGTM. I plan to rename datastore.dataset.Dataset -> datastore.client.Client after this merges.

[dhermes]

OK SGTM