Handling for extra args in _audit method.

- Move non-reserved kwargs into extra args. This make the audit method easier to use.
Include python-json-logger
Update logging.conf to use JsonFormatter.
Update audit calls to use structured logging.
Add more fields to audit log line.
to_serializable_dict method in stormbase

- This method returns a serializable dict of the data fields of a db model object. This output of this method can be passed to json.dumps(...)
Use to_serializable json.
Add json formatted audit log handler in syslog config.
Explicitly use extra rather than generating in the _audit method.
Merge pull request #814 from StackStorm/json-audit-log

Audit logs in JSON