Update to new message format - AGPUSH-1046

Updates to the new message format, https://issues.jboss.org/browse/AGPUSH-534

@sebastienblanc @edewit  mind taking a look
custom keys should be inside an object called `user-data` see: [new-format](https://github.com/aerogear/aerogear-unifiedpush-server/blob/db9b52cd22bc7d5f9bd4899ab275ebefb4cea78d/jaxrs/src/test/resources/new-message-format.json)