Update Key API to be more user-friendly per design doc discussion

[dhermes]

Related to #3 and
https://docs.google.com/document/d/1xKw8Tz6lfN5uOcv0my4uUgFSeDoRzwCLho2UGlBpqDU/edit

Plan to make this work:

• Remove Key.from_path (since will be new constructor) (Address first part of 451: Remove Key.from_path #456)
• Remove all setters (Address second part of 451: Remove all setters and make attrs into @property #458)
• Make parent re-use an instance (since immutable) (Address third part of 451: Storing Key.parent on instance for future calls #459)
• Make Key constructor act like Key.from_path (Address fourth part of 451: Make Key constructor easier to use by taking positional args. #460)
• Make Key.complete_key method (needs better name) to act like Key.id(new_id) (Address fifth part of 451: Implement Key.complete_key for auto_id on partial keys. #461)
• Implement Key.compare_to_proto to check a key against a retrieved key (needs better name) (Address sixth part of 451: Implement Key.compare_to_proto to check pb keys against existing. #462)
• Require dataset_id on Key (may require removing dataset_id from `Key protos) (Address seventh part of 451: Make dataset_id required on Key. #463)
• Add parent to constructor (Address eighth part of 451: Add parent to constructor #476)
• Update docstrings and other docs to reflect changes (including __repr__, probably shouldn't have path) (Putting off bulk doc fixes until API is stable again)
• Change Entity constructor to only accept a key. (This can probably move to a new bug, which I have yet to file.) (filed Remove dataset from Entity constructor and allow a key #484)