Add Firebase Messaging support (which replaced GCM)

[techscientist]

Hi,

Recently, Google-acquired Firebase announced Firebase Cloud Messaging (FCM) as the new alternative to Google's GCM. As such, I think we should replace gcm in this library with firebase messaging.

https://firebase.google.com/docs/cloud-messaging/

I've found a python library for fcm: https://github.com/olucurious/PyFCM

What do you guys think?

[rakanalh]

Hi @techscientist,

You're right. We should support FCM but without removing GCM, but rather marking it as deprecated so that people can still use it at their own risk. Would you like to have a go at this and integrate firebase? contributions are very welcome!