CAMediaTiming Protocol
CALayre 和 CAAnimation 实现了CAMediaTiming 接口。
CAMediaTiming 定义了8个属性。
speed属性：
Controlling the speed of an animation or a layer can also be used to pause the animation
by setting the speed to 0.
将speed设置为0可以来暂停动画。
下面这个Demo演示speed的使用：
https://github.com/d-ronnqvist/blogpost-codesample-PullToRefresh
Reference:
1. Controlling Animation Timing
http://ronnqvi.st/controlling-animation-timing/