code: function createAnimitter(options, fn){
  
      if( arguments.length === 1 && typeof options === 'function'){
          fn = options;
          options = {};
      }
  
      var _instance = new Animitter( options );
  
      if( fn ){
          _instance.on('update', fn);
      }
  
      return _instance;
  }
docs: create an animitter instance,
  @param {Object} [options]
  @param {Function} fn( deltaTime:Number, elapsedTime:Number, frameCount:Number )
  @returns {Animitter}