gpl: added a license file for GPLv3

The COPYING file contains the full GPLv3 license, the license this
project is licensed under.

ETA/OS  Copyright (C) 2014  Michel Megens <dev@michelmegens.net>
    This program comes with ABSOLUTELY NO WARRANTY; for details type
    `show w'. This is free software, and you are welcome to redistribute it
    under certain conditions; type `show c' for details.

Signed-off-by: Michel Megens <dev@michelmegens.net>