Correct checking/handling for push tokens
Rough handling of incoming push notifications

Needs more work, but the basics are in place.
Optimisation
Refactoring: AuthenticationService
Refactoring: GravatarService
Do not synchronise when setting last update

It isn't that necessary.
Reregister for remote notifications when becoming active

This way we always deal with fresh data, see https://www.pushlayer.com/blog/2013/03/12/ios-push-notifications-best-practices