Fix logging

Making logging consistent between web.py and guardian.py:
* Use the the same file handler for the standard logger and app.logger;
* Replace calls to app.logger with logger;
* Add backupCount to rotate logs.

In order to use the logging module directly in this and other files, we should add this handler to the root handler, that is, logging.getLogger(). Also I think that the logger.setLevel() is unnecessary since we set it on file_handler above. So we can just call addHandler() all in one line, i.e., logging.getLogger().addHandler(file_handler).

But please test this to make sure I'm right. :)