Ensure records loaded after a FilteredRecordArray is created are sent through the filter

I was finding that if I created a FilteredRecordArray and then subsequently loaded records, they were not being included by the filter as expected.

The fix to the initial problem is to ensure loaded records are added to any record arrays.

While I was poking around I also noticed that Ember.Model was aware of Ember.FilteredRecordArray, so I cleaned that up a bit as well. This can be moved to another PR or dropped altogether without affecting the actual fix.