[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
Actually the signature would be _audit(logger, msg, *args, **kwargs)

call would be
`log.audit('my message', extra={'actionexecution': actionexecution_db})` 
as opposed to 
`log.audit('my message', actionexecution=actionexecution_db)`