Ember.Model doesn't need to know about FilteredRecordArray

Updated PR to replace #209. Added FilteredRecordArray#removeObject() to clean up the observers.

There was a question of whether pushObject() should raise an exception if the supplied record is not of the correct type. To my thinking this is no different than if the model is of the correct type but the filter does not match - ie. the client calls pushObject() and the supplied object is not added to the FilteredRecordArray.