Generating Firebase authentication tokens for logged-in users.
Authenticating the backend with Firebase (with admin rights).
Better handling of user photo thumbnails.

-) taking any of the 3-sized pics from RunKeeper.
-) showing default if none exists.
-) re-sizing up to maximal size.

Fixes #11.
Logging out even if connection to the server dropped.

Plus un-authenticating from the Firebase data source.