Emit 'exit' event and transition to standby when app terminates

All connected web clients will show the correct UI state
and will display a helpful error.
Move exit state action into gracefulExit handler
Bind HTTP API to shutdown and restart actions
Display shutdown messages in web UI
Physical UI now uses shutdown state action
Merge pull request #11 from radiodan/transition-to-standby-on-exit

Emit 'exit' event and transition to standby when app terminates