Support Safari Push and Mobile Device Management

Rounding out support different types push notifications:

* Title, Action and URLArgs are for Safari Push notifications.
* MDM is for Mobile Device Management.

This just allows these types of notifications through, it doesn't ensure that they are fully valid. I'm doing validations in advance.
That's odd... I'm guessing it's something else. http://play.golang.org/p/lGAUCTAjGz