Mixin for Backbone.Model added

Usage example:
var Book = Backbone.Model.extend({
  initialize: function(){
    Swarm.BackboneModelMixin.call(this, 'Book');
  }
}
bb draft
Backbone mixins done

How it works you may see here -
https://github.com/swarmjs/todomvc-backbone-swarm/commit/27c80dfa7c5d1178574e9aca7b6e932461c022f5
Merge pull request #1 from swarmjs/feature/backbone-model

Backbone model & collection mixins