Fixes relational model issue with player/pieces, add playability

* Pieces weren't having their 'player' attribute set due to an
  issue between Backbone.Relational and Require.js. Fixed in
  model/pieces.js using addModelScope.
* Adds playabiity to the game by allowing the baord to fire movement
  events.
* Extends defaults of models from Base piece properly (was being
  overwritten entirely before).