Use roar instead of active_model_serializers

[Roar](https://github.com/apotonick/roar-rails) is a gem that allows us to easily serialize out database objects into JSON format, while offering us easy integration of [hypermedia](http://restcookbook.com/Basics/hateoas/) in our API.

The format of the JSON output will be enhanced now, which may or may not break current Angular factory, although I suspect it won't!