code: function isLoaded (url) {
          if (urls[url] == true) {
              return true;
          }
  
          var image = new Image();
          image.src = url;
          if (image.complete) {
              return true;
          }
  
          return false;
      }
docs: the urls currently being loaded. Each key contains an array with callbacks 
  Check if an image url is loaded
  @param {String} url
  @return {boolean} loaded   True when loaded, false when not loaded
  or when being loaded