Remove Dataset.get_entity and move the functionality to Key. See https://docs.google.com/document/d/1xKw8Tz6lfN5uOcv0my4uUgFSeDoRzwCLho2UGlBpqDU/edit https://github.com/dhermes/gcloud-python/tree/move-get-to-key
