code: function(handler, priority) {
          if(handler && handler.constructor === Array) {
            handler.forEach(function(layer) {
              layer.priority = typeof layer.priority === 'undefined' ? 1 : layer.priority;
            });
            _list = _list.concat(handler);
          } else {
            handler.priority = priority || (typeof handler.priority === 'undefined' ? 1 : handler.priority);
            _list.push(handler);
          }
          return this;
        }
docs: Add a handler
  
  @param {Mixed}  handler A Function or an Array of Functions to add to the queue
  @return {Queue}         Itself; chainable