code: function(callback) {
          var anim = this;
  
          if (!anim._running) {
              anim._running = true;
  
              anim._node.fire('transition:start', {
                  type: 'transition:start',
                  config: anim._config
              });
  
              anim._start();
              anim._callback = callback;
          }
  
          return anim;
      }
docs: /*
  Starts or an animation.
  @method run
  @chainable