UIWebView左右划动翻页手势
最近需要在UIWebView里实现左右划动翻页的功能，发现用UITouch事件没效果，在官方文档里找了个用UIGestureRecognizer实现的方法：
UITapGestureRecognizerUIPinchGestureRecognizerUIRotationGestureRecognizerUISwipeGestureRecognizerUIPanGestureRecognizerUILongPressGestureRecognizer
