Expand EntityReadMixin

Make `EntityReadMixin` provide the following methods to inheriting classes:

* `read`
* `read_json`

The `read` method acts just as before. The new `read_json` method does just what
you'd expect: read information from the server and return decoded JSON.

Add tests for the new `read_json` method.
Merge pull request #1268 from Ichimonji10/larger-read-mixin

Expand EntityReadMixin