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.

Now that we have the DEBUG config variable, which is used in both web.py and guardian.py, I think we should remove the command-line option from guardian.py. It's particularly confusing for the default to change depending on the config.py, so let's just use config.py.