This change address #274 to add 'verbose option which outputs minimum information (for usual runs with a bit information than 'error').
This changes adds logging message:debug and error. fmt.Errorf() is wrapped by logging.Errorf() hence all error message is also put to log file. logging.Debugf() is called at almost function call, so we could track the code by logging message.
This diff adds logging feature for debug.
