Model Cleanup

Bringing us some sexy models. Not ready for merge yet.

#### Done
* Added communication layer for caching and testing called adapters, to stay conceptually similar to Ember Data
* Created two adapters, AjaxAdapter and FixtureAdapter. In the future we'll probably add some sort of CachingAdapter
* Got tests working using FixtureAdapter
* Add isLoading property so users can observe for when models are populated
* Implemented hasMany to lazily fetch associations using URIs

#### To-Do
* Support CRUD actions for models (well - we have read already, so mostly just CUD)