Updates reference and referenceCache when model primary key changes.

1. Model.create will create a reference with the current primary key (in my use case, this is undefined because the primary key is set by the server on POST).
2. The response from the POST is then loaded into the model (now with primary key defined). CreateReference should be updated with the new attributes and the referenceCache should add an entry for this primary key, so Ember.Model.find can find the record later.

Thanks!