add 3 tests and fixes:

- transforms are now applied
- camelCased attributes and relationships in `DS.Model`s are correctly converted to/from underscored JSON payload names
- camelCased `DS.Model` names are converted correctly to/from underscored URLs

These are fixed by by extending `DS.RESTSerializer` instead of `DS.JSONSerializer` which allows us to steal the built in [`normalize` method](https://github.com/emberjs/data/blob/master/packages/ember-data/lib/serializers/rest_serializer.js#L120) which exposes a way to define the desired attribute/relationship key modifications.