NSJSONSerialization-NSNullRemoval在NSJSONSerialization、NSMutableDictionary和NSMutableArray中创建类别，用于递归删除JSON服务返回的NSNull对象。许可证: MITObjective CUIView-AutolayoutUIView的类别，简化常见布局约束的创建。许可证: MITObjective C
