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.
```cls``` and ```self``` have different meanings in Python. ```self``` is used to reference instance variables and ```cls``` class variables. When using the ```@classmethod``` decorator, the first argument to the method must always be ```cls``` and not ```self```