让CCLayer响应ccTouchBegan而非Touches

In issue 652, a project member said:

Layer need to implement both methods: ccTouches and ccTouch.
The default one is ccTouches but you can modify the default behavior by overriding 
the "registerWithTouchDispatcher".

具体做法就是在CCLayer子类中增加如下代码：

-(void) registerWithTouchDispatcher {
    [[CCTouchDispatcher sharedDispatcher] addTargetedDelegate:self priority:0 swallowsTouches:YES];
}