gpe-expenses moves to GPLv3

I actually should have done this at the last release as that already includes some files licenced under GPLv3 but now I'm ready to make an upstream release (including an updated German translation that missed the original string deadline). The only real change is that debian/copyright has grown to the extent that each package is 10kb larger than before!
One day, when GPL3 goes into /usr/share/common-licences, I'll get back to the small packages. Still, Emdebian won't need the extra bloat so it's only a Debian issue.