UIScrollView-工具条悬停和下拉放大图片效果

demo

http://od2d96feb.bkt.clouddn.com/%E5%B7%A5%E5%85%B7%E6%9D%A1%E6%82%AC%E5%81%9C%E5%92%8C%E4%B8%8B%E6%8B%89%E6%94%BE%E5%A4%A7%E5%9B%BE%E7%89%87%E6%95%88%E6%9E%9C2016-11-30%2013_02_24.gif