Docstrings and Unit Tests

- [x] Adapter docstrings
- [x] Serializer docstrings
- [x] Install sinon for stub/mocking
- [x] Adapter unit tests
- [x] Serializer unit tests

/cc @holandes22 @benkonrath
Yes, this is intentional.  I looked at how ember-data does it and mimicked their behavior:

Here: https://github.com/emberjs/data/blob/master/.jshintrc#L65

And, for example, here: https://github.com/emberjs/data/blob/master/packages/ember-data/lib/adapters/rest_adapter.js#L440-L445 (note how `relationship` is not used)