Add support to log circus messages to syslog.

Add the possibility to configure the Python logging infrastructure to log in syslog instead in a file. Update circusd-stats to use the same configuration mechanism than circusd. As well when creating a watcher for circusd-stats, inherit the logging setting from the circusd configuration.

Unfortunately, there are no test for the configuration of logging and to be honest, I don't see how to test this.