Binary notification format (closes #35)

This change allows Pushy to use the newer APNs binary notification format. The only real user-facing change here is that push notifications can now have a delivery priority.

For reviewers, we're moving from the ["Enhanced Notification Format"](https://developer.apple.com/library/ios/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/Chapters/LegacyFormat.html) to the ["Binary Notification Format"](https://developer.apple.com/library/ios/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/Chapters/CommunicatingWIthAPS.html). I added some notes about corner cases to [the wiki](https://github.com/relayrides/pushy/wiki/Development-notes).

TODO:
- [x] Update docs/copyright headers (ef37154)
- [x] Decide how to handle `null` delivery priorities (8bc8904)