CHAnimation 介绍
CHAnimation 用来演示如何编写自己的动画引擎，灵感来自于 Facebook Pop，只需 600 行的 Objective-C 代码。
使用方法：
#import "CHAnimation.h"
CHAnimation *animation = [CHAnimation new];
animation.duration = 0.4;
animation.writeBlock = ^(id obj, id value) {
button.center = [value CGPointValue];
};
animation.fromValue = [NSValue valueWithCGPoint:CGPointMake(70, 70)];
animation.tovalue = [NSValue valueWithCGPoint:CGPointMake(170, 170)];
[button ch_addAnimation:animation forKey:@"animation"];