[WIP] Audit logs in JSON

See https://gist.github.com/manasdk/50cf0f97c069da131274 for a sample of an audit log file generated by st2actions.

- Audit logs in JSON so that they can be easily parsed by logstash, splunk etc.
- Using https://github.com/madzak/python-json-logger as the log formatter. May write own JSONFormatter or contribute to python-json-logger if they are amicable with my choices.

## TODO

- [ ] Fix regular logs where the extra fields are ignored. - Manas
@Kami: Are you saying audit method should have explicit kwargs like

_audit(logger, msg, *args, extra=None, exc_info=False, **kwargs)