Subscribe to tap change events.

As a side-effect, the event’s Producer will cause
`updateTapDetails()` to run immediately after registration in 
`onStart()`, avoiding a race condition which could otherwise
leave the tap detail unset.
Avoid spurious updates.
Update unknown drinker image to match kegbot-server.
Remove unconditional immediate checkin.
Add fragments statically.
Treat empty username as anonymous pour.
Show 2 taps at a time on home screen.
Touch tap to authenticate; move registration off main screen.
Add immersive mode for testing; default disabled.
Show tap description when no image available.
v1.0.0-b76