[PoC] support both logrus logger and entity

[makasim]

I am embedding Guerrilla SMTP server into my code and there is a need to pass Logrus Entry as a logger to the daemon. The problem is that HookedLogger accepts a Logger struct so it is not possible to pass Entry. Consider next code

        d := guerrilla.Daemon{}
	if err := d.SetConfig(cfg); err != nil {
		return nil, err
	}

	smtpLogger, err := guerrillaLog.GetLogger(LogFile, LogLevel)
	if err != nil {
		return nil, errors.New("cannot init guerrila logger")
	}

	hookedLogger, ok := smtpLogger.(*guerrillaLog.HookedLogger)
	if !ok {
		return nil, errors.New("guerrila logger must be hooked logger")
	}
	hookedLogger.Logger = logger // logrus.Entry
        // works atm but I lost fields associated with entry
	// hookedLogger.Logger = logger.Logger 
	d.Logger = hookedLogger

	return &Server{
		daemon: &d,
	}, nil

I am going to add tests and fix code review comments if it makes sense to you in general and is going to be merged. Just let me know.