irkerd: Convert to Python's logging module

irkerd: Convert to Python's logging module

Instead of using the local IRCClient.debug() method, use the more
flexible standard library logger.  This makes it easy to log to
syslog, rotating files, etc, using the usual logging Handlers.  The
mapping from the old implementation to the new implementation is:

  IRCClient.debug(1, message)  -> LOG.info(message)
  IRCClient.debug(2, message)  -> LOG.debug(message)
  IRCClient.debug(50, message) -> LOG.debug(message)
  Irker.logerr(errmsg)         -> LOG.error(message)

with the exception of the failed-message error, which is logged as
LOG.warning().  I didn't try and recategorize the other message log
levels, although I think a number of info-level log messages should
really be debug-level log messages.

To set the log level, the -d option now takes string arguments
(e.g. 'info', 'debug') instead of numeric arguments (e.g. '1', '2').
This breaks backward compatibility, but I think it makes the argument
more user-friendly.  If you try and set an invalid level, there's a
helpful error message to guide you in the right direction.

I also use format_exc() in Connection.dequeue (following the existing
example deeper in the Connection.dequeue nest).  The log level should
decide whether the traceback is printed or not, not whether the
exception should be raised or ignored.