DISCUSSION: Implement Key.__eq__ and Entity.__eq__

Currently comparison must be done in an ad-hoc way:

assert dict(arya_entity.items()) == {'name': 'Arya', 'family': 'Stark'})

FWIW __eq__ is not something that should really be needed outside of the Key, and even then not so much.

NOTE: This was a small part of #336

FWIW I have implemented Key.__eq__ in https://github.com/dhermes/gcloud-python/tree/key-comparison but it greatly depends on the conversation in #528

Can we simplify by saying that incomplete keys never compare equal to anything? We shoudn't expect them to be super long-lived anyway, and the use-cases seem strongly biased toward "real" equivalence.

Note that we need to implement __hash__ as well for many sensible uses of __eq__.

@dhermes

Just had a chat with @dhermes about "what does equality mean?". The current thinking is:

Equal key path
Equal entity data (dictionary equivalence)
Equal dataset_id (if dataset_id.startswith(s~ or e~): dataset_id = dataset_id[2:])

If people just want to compare the entity data, they can do a dict(entity) ==
If people just want to compare the key path, they can do a key.path ==