swap `Ember.Set` for `Ember.NativeArray` - closes #20

As of Ember 1.8, `Ember.Set` will be deprecated. `Ember.NativeArray`'s
`#addObject` and `#removeObject` will only add if the item isn't found
in the array and will remove all occurances.

See http://emberjs.com/api/classes/Ember.NativeArray.html#method_addObject
and http://emberjs.com/api/classes/Ember.NativeArray.html#method_removeObject for more.