解决requestAnimationFrame的兼容问题

写法：

1 window.requestAnimFrame = (function () {
2         return window.requestAnimationFrame ||
3             window.webkitRequestAnimationFrame ||
4             window.mozRequestAnimationFrame ||
5             function (callback) {
6                 window.setTimeout(callback, 6000 / 60);
7             };
8     })();