Xtrabackup Manager - License changed from GPLv3 to GPLv2

Hi Folks,

I changed the license on Xtrabackup Manager from GPLv3 to GPLv2 today. This should make it more compatible with the MySQL Ecosystem.

If anyone has any objections, advice, comments -- please let them fly :)

Meanwhile, testing and work continue...

Cheers,
Lachlan