Support for APNS alert dictionary schema

[timbotnik]

Currently the alert only supports a single string field, however the APNS alert field can be expanded to be either a string OR a dictionary as in:

aps: {
  alert: {
    title: "some title",
    body: "some body",
    ...
  }
}

As documented here: https://developer.apple.com/library/ios/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/Chapters/ApplePushService.html#//apple_ref/doc/uid/TP40008194-CH100-SW1

Our immediate need is for title, body and launch-image support, but it might make sense to handle all of the localization keys as well while you're at it.

[Tim-W]

+1

[raix]

This would require some refactoring, but a good idea