遵守了 UIScrollViewDelegate
划到那一页，标签 显示 对应的页码
func scrollViewDidEndDecelerating(scrollView: UIScrollView) {
let index = Int(scrollView.contentOffset.x/scrollView.bounds.size.width)
self.photoIndex = index
self.titleLabel!.text = "第\(self.photoIndex! + 1)页， 共\((self.photoArray?.count)!)页"
}
