PM::Delegate improvements, PushNotification object & methods

The PushNotification stuff needs to be tested on a real device to make sure it works (can't test on the simulator), but the new PM::Delegate is a lot nicer code (there were some backflips we were doing there that was left over from when Screens weren't UIViewControllers).

Documentation:

https://github.com/clearsightstudio/ProMotion/wiki/API-Reference:-ProMotion::Delegate
https://github.com/clearsightstudio/ProMotion/wiki/Usage:-Push-Notifications

I'll be gone this weekend, but Silas and Matt can review, comment, and/or merge.