AutoLayout 代码添加约束

用NSLayoutConstraint添加View的Constraint

1.View的translatesAutoresizingMaskIntoConstraints设置为NO；
2.constraint对象的active设置为Yes，默认为NO；
3.必须添加View到superView；
4.对superView添加constraint。
如果对View添加constraint，会报错如下：
Terminating app due to uncaught exception 'NSGenericException', reason: 'Unable to install constraint on view. Does the constraint reference something from outside the subtree of the view? That's illegal.
需要对superView设置constraint，不是对View设置constraint