Divij/client message flow

Wiring up application registration with client "init_interview" message.

For the time being we are hardcoding registered applications see line 52, 53 on interview.py. Registration via selection screen to follow.

PEP8 Compliant. Tested notes and heartbeat application.
setup_apps seems like a strange name for this since it just notifies all apps that someone has joined the interview. I'd probably call it something like notifiy_apps_client_joined or something like that