Added `fallbackSelector` and `loadingSelector` to constructor instance options

- both `fallbackSelector` and `loadingSelector` are useful when not subclassing CollectionView.
- dont need a template on the CollectionView to work.

ex:
```coffeescript
    @subview 'items', new CollectionView
      region: 'items'
      noWrap: yes
      itemView: MyItemView
      collection: @items
      fallbackSelector: '.no-items'
```