Fixes to drag and drop for listview
- Updated @_draggedItems to be a collection, similar to @_selected so we can
  trigger events as needed
  - Add explicit drop trigger on draggedItems
- Add data-model-id information so we can perform searches through a
  listview's items and easily return by model id
- Use Backbone.Collection.contains for model inclusion test.
Merge pull request #44 from rickbenavidez/listview-dragdrop

Fixes to drag and drop for listview