Remove Dataset.get_entity and move the functionality to Key

[dhermes]

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

[dhermes]

Will be handled in #477 fixes