Add Safari Push Notification support

I added "Notification#url_args" to define the url_args that are send to the APNS Service.
The url_args are required for [Safari Push Notifications](https://developer.apple.com/library/mac/documentation/NetworkingInternet/Conceptual/NotificationProgrammingGuideForWebsites/PushNotifications/PushNotifications.html).

Issue: https://github.com/rpush/rpush/issues/76