Fix #133:  preserve PB namespace in Key.from_protobuf.
Fix #134:  enforce contract that *args must be pairs.
Propose a contract for Key.parent.
Fix #135:  enforce contract for Key.parent().
Merge branch 'add_key_docstrings' into 133-key.from_protobuf-preserve-namespace
Merge branch 'add_key_docstrings' into 134-key-from_path-contract
Merge branch 'add_key_docstrings' into 135-key-parent-contract
Incorporate feedback from @dhermes.
Merge branch 'add_key_docstrings' into 135-key-parent-contract
Merge pull request #183 from tseaver/133-key.from_protobuf-preserve-namespace

Fix #133:  preserve PB namespace in Key.from_protobuf.
Merge pull request #185 from tseaver/134-key-from_path-contract

Fix #134:  document / enforce Key.from_path contract
Merge pull request #186 from tseaver/135-key-parent-contract

Fix #135:  Document / enforce gcloud.datastore.key.Key.parent() contract