Use logger instead of `plog`

This replaces plog. In main() you see an example of how you
could use log-levels and the verbosity-flag from commandline.

If you really want to discriminate even further within a module
or across the project, you could use named dedicated loggers, e.g.::

    logger_net = logging.getLogger('network')
    logger_sys = logging.getLogger('system')

IMHO log-levels and logger by __name__ should be sufficient, though.