Add MINotification Category for NSObject

The API of Notification is a litter ugly, so we add a category for NSObject to simplify the API. You can see the API in the NSObject+MINotification.h file

Add callback for the MIViewController

In v1.0.0, we didn't handle the callback for a scene, and just set a temp parameter in the method -viewControllerWithScene:context:callback:(MICallback _Nullable)callback; of the MIMediator. In this version, we store the callback block in the MIViewController to send data back to the previous View Controller.