Using tornado.general logging stream instead of the default one

Sockjs-tornado uses the default logger from `logging` module and it will therefore override pretty log format set by Tornado on startup. This commit makes the application code log messages to the `tornado.general` stream, thus preserving the log format (and behaving coherently with Tornado).

Please find an explanation of logging in Tornado [here](http://www.tornadoweb.org/en/stable/log.html#module-tornado.log)