Use Ember.copy() for model.copy()

Ember.copy() also correctly copies arrays, objects and such (i.e.
embedded records). Without Ember.copy(), only the references get copied,
and updates on the the original still have an effect on the copy and
vice versa.

This fixes issue #21.
Test for deep copy in Model.copy()
Merge branch 'hperl_model_copy_fix' into tees