[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
I would be good to add some kind of comment here since I was a bit confused about what is going on here at first.

Anyway, it looks like python json logger / formatter just assumes all the additional payload which is to be JSON formatted is included in the `extra` key so we shove everything except "special" logging kwargs to the extra dict.