logging should be unified

[ColinDKelley]

Currently the code logs 4 different ways:

1. Listen.logger.info "message"
2. Listen::Logger.info "message"
3. _log :info, "message"
4. self.class._log :info, "message"

These can all be unified with (1).

Also, there is a comment about clearing the logger, but the code doesn't actually support an empty (nil) logger. Instead, we can just have a comment describing how the caller can point the logger to /dev/null if they don't want logging.