code: function SpriteSheetLoader(url, crossorigin) {
      /*
       * i use texture packer to load the assets..
       * http://www.codeandweb.com/texturepacker
       * make sure to set the format as 'JSON'
       */
      EventTarget.call(this);
  
      /**
       * The url of the bitmap font data
       *
       * @property url
       * @type String
       */
      this.url = url;
  
      /**
       * Whether the requests should be treated as cross origin
       *
       * @property crossorigin
       * @type Boolean
       */
      this.crossorigin = crossorigin;
  
      /**
       * [read-only] The base url of the bitmap font data
       *
       * @property baseUrl
       * @type String
       * @readOnly
       */
      this.baseUrl = url.replace(/[^\/]*$/, '');
  
      /**
       * The texture being loaded
       *
       * @property texture
       * @type Texture
       */
      this.texture = null;
  
      /**
       * The frames of the sprite sheet
       *
       * @property frames
       * @type Object
       */
      this.frames = {};
  }
docs: The sprite sheet loader is used to load in JSON sprite sheet data
  To generate the data you can use http://www.codeandweb.com/texturepacker and publish the 'JSON' format
  There is a free version so thats nice, although the paid version is great value for money.
  It is highly recommended to use Sprite sheets (also know as texture atlas') as it means sprite's can be batched and drawn together for highly increased rendering speed.
  Once the data has been loaded the frames are stored in the texture cache and can be accessed though Texture.fromFrameId() and Sprite.fromFromeId()
  This loader will also load the image file that the Spritesheet points to as well as the data.
  When loaded this class will dispatch a 'loaded' event
  
  @class SpriteSheetLoader
  @uses EventTarget
  @constructor
  @param url {String} The url of the sprite sheet JSON file
  @param crossorigin {Boolean} Whether requests should be treated as crossorigin