Added Nexuiz-2.5.2.

I have added Nexuiz-2.5.2.

possible optional TODO's:
- split the DarkPlaces engine so games using it (Nexuiz, Xonotic and others) would not need to compile it seperately. I tried but encountered many problems in both Nexuiz and Xonotic, I might give it a shot some other time again.
- add libOffscreenGecko to nixpkgs and add it to the rpath as Nexuiz searches for it at runtime.