DISCUSSION: Implement Key.__eq__ and Entity.__eq__

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

``` python
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
