[WIP] Remove Ember.DeferredMixin from model and RecordArray

I've took a stab at this to see how much thins will need changing. I went with `Ember.loadPromise` implementation, which wraps any object, which uses `isLoading` and `didLoad` event. This seems a nice approach, because any object following such contract will work out of the box. The only issue is that because of the model's specifics I need to check for `isNew` as well, which makes `LoadPromise` more tightly coupled with model.

This PR may wait until a related API is settled for ED as well, so EM can stay compatible.