New Feature: PushNotifications for OSX

Starting with commit b8b304e you will be able to use notifications on OSX too. The demo app has been updated to work with OSX as well.

As mentioned in the demo, you need to make sure that your app is configured in Apple Developer Centre - push notifications capability needs to be turned on and the certificate needs to be generated and uploaded (follow instructions in the dev centre). This config step applies both for iOS and OSX.