Propose a contract for Key.parent.
Fix #135:  enforce contract for Key.parent().
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 #186 from tseaver/135-key-parent-contract

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