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.
This is a basic method that accepts absolutely anything via ```*args``` and ```**kwargs```. Ideally, any application that needs access to the socket can specify its the arguments in its own ```on_join``` method.