Python client for FCM - Firebase Cloud Messaging (Android, iOS and Web).

Firebase Cloud Messaging is the successor to GCM, inheriting its
infrastructure and adding new features.  Using FCM, an application can
notify a client app that new email or other data is available to sync,
and can send notifications to drive user reengagement and retention.
For use cases such as instant messaging, a message can carry a payload
of up to 4KB to a client app.
