当viewcontroller比较多的时候，可能会忘记给storyboard的viewcontroller指定class，就会报下面这样错误：

 *** Terminating app due to uncaught exception ‘NSUnknownKeyException‘, reason: ‘[<UITableViewController 0x1092c5440> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key words.‘

解决方法就是给viewcontroller指定class