设置缓冲

使用 setEasing 方法可以设置动画缓冲，比如：

action.setEasing(Bling.TWEEN.Easing.Quartic.InOut);

所有内置缓冲如下：

Linear.None
Quadratic.In	Quadratic.Out	Quadratic.InOut	Cubic.In	Cubic.Out
Quartic.In	Quartic.Out	Quartic.InOut	Quintic.In	Quintic.Out
Sinusoidal.In	Sinusoidal.Out	Sinusoidal.InOut	Exponential.In	Exponential.Out
Circular.In	Circular.Out	Circular.InOut	Elastic.In	Elastic.Out
Back.In	Back.Out	Back.InOut	Bounce.In	Bounce.Out