Add a Flag to remove all debug

[buzzcola3]

It would be nice to have a flag that removes all debugging/serial output.

In my project, the serial is wired to a 3d printer

[Renzo Mischianti]

Ops... sorry I forgot to comment this line
https://github.com/xreef/SimpleFTPServer/blob/master/FtpServerKey.h#L21

I must redò a new version.

Now comment that line.

bye Renzo

[buzzcola3]

Nwm, found it