code: function( assetsToLoad, callback ){
  
              if ( !Physics.util.isArray( assetsToLoad ) ) {
                  throw 'Spritesheets must be defined in arrays';
              }
  
              var self = this
                  ,loader = new PIXI.AssetLoader(assetsToLoad)
                  ;
  
              // Start loading resources!
              loader.load();
  
              loader.on('onComplete', function(evt){
                  self.assetsLoaded = true;
                  callback();
              });
  
              return self;
          }
docs: PixiRenderer#loadSpriteSheets( assetsToLoad, callback ) -> this
  - assetsToLoad (Array): Array of spritesheets to load
  - callback (Function): Function to call when loading is complete
  
  Loads textures defined in a spritesheet