Firebase.APNS Support

[cbaggers]

Firebase push notifications are great on android but weak on ios. The reason is that a firebase push notification cant fire on iOS if the app is in the background or closed. APNS notifications can do this (and is supported through Fuse.APNS) but that means managing that second backend (and its a pain in the ass).

Firebase has support for delivering notifications via APNS, which has all the benefits of APNS but with the benefit that Firebase is a way easier backend to talk to.

The goal is to add support for this to Fuse.Firebase.

I'd like to do this without disturbing the current Firebase.PushNotification support. Which, whilst there is some api overlap, I think is possible as no project is going to use both methods of talking to iOS at once.

[cbaggers]

Im going to work on this this weekend. It should be pretty easy. I'm also going to bring the push-notify-topics branch into this too and try and kill both birds with one stone.

[LuisRodriguezLD]

With help from @devadiab I've been experimenting with this and we got some nice results. #82 might help you :)

[cbaggers]

@LuisRodriguezLD oo sweet, where would you say the feature is at an what still needs doing?

[LuisRodriguezLD]

I would say testing (I made some tests and they all worked but still) and maybe a clean up. I am no uno master but I get the feeling the some code is unnecessary.

Also, push-notify-topics would be a great addition :)

[cbaggers]

PR is here #106