缓慢加载图片的动画,效果来源： http://static1.squarespace.com/static/5551190fe4b03147c4b32f55/t/55522723e4b0a69c0915b5bf/1431447440480/?format=500w
##如何使用
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cocoapods导入：
pod 'DDRevelImage', '~> 1.0.1'
##效果预览
##参考资料 http://oldyellowbricks.com/blog/transitions-with-coreanimation-revealing-image