Fix build of Debian package

I'm new to the whole github thing, so I want to start of with something small and simple:

The build of the Debian package had some issues:
* AUTHORS, CHANGELOG and COPYRIGHT files were placed in /usr/lib/openra
* /usr/share/doc/openra/changelog.gz did not contain the changelog, because the script contained the wrong path to the CHANGELOG file
* Due to the above, the build failed on my system and I was not able to build a .deb

I fixed that by placing AUTHORS and CHANGELOG in /usr/share/doc/openra and removing the COPYRIGHT file (the license texts are located in /usr/share/common-licences on Debian-based systems).  The build now also finishes successfully on my system.