Use cpack to create osx's dmg

Additionally, ensure proper installation of sounds and zonebg resources.
This by now is needed in order to create a working OSX package, but can be a base to create linux packages (deb/rpm) or a nsis script for windows in the future.
I'll change to the defined. Generally you can just send a pull request. For something this simple you can do it all in github ui. (fork, edit file in web browser, commit, send PR)