Source tarball releases for code+data and data-source

[apoleon]

I think the current tarball release for caveexpress and cavepacker is adequate as soon as

#10

is resolved. I suggest to improve the other tarball, contrib-assets-png.tar.xz, by adding all artwork assets to it (compressed gimp files and others). This will increase the download size to approximately ~500 MB. Nevertheless I think this will be very convenient for users who want to modify artwork and contribute to additional levels for caveexpress and cavepacker. If such a tarball with artwork sources existed, I would prominently promote this one in the Debian package of caveexpress and cavepacker. Besides the "artwork-source-tarball" would rarely need updates hence I think it is maintainable.