Add node-bunyan as a dependency

Take a look at https://github.com/trentm/node-bunyan for more informations
Logs commons signaling steps

- Sign in
- Sign out
- WS Connections
- WS Disconnections
- Calls initiations
- Calls answers
- Calls hangups
Add a LOG_LEVEL config option
Do not log src lines anymore
Merge branch 'master' into server-logs
Merge pull request #171 from tOkeshu/server-logs

Server structured logs