Dont propagate log entries to higher level loggers

If modules use crumbs and also use the standard Python logging module, log messages from crumbs will propagate to the higher level logger unless propagate is disabled. IMO leaving propagate enabled is unexpected and undesirable, especially since configuration files often contain sensitive information (username, password, etc.).

As a work around I've added `logging.getLogger('crumbs').propagate = False` to the project I'm working on now.