Added directive to display number of watchers on the page.
Greatly reduced number of watchers for workouts' list with bindonce.

On exemplary data (my 2013 workouts) number of watchers reduced from 8777 to 1620.
DataService -> WorkoutsProviderService
Big re-factoring of AuthService and DataService.

-) AuthService now provides listeners for user changes (instead of synchronizing user via $rootScope).
-) DataService split into DataProviderService and WorkoutProviderService; first step towards having a centralized storage management (to have multiple Firebase profiles).

Fixes #7.
Allowing Firebase root to be specified via a flag (FIREBASE_ROOT).

Hence allowing separate dev/prod Firebase profiles.