Added nitrogen package

I packaged nitrogen, a wallpaper utility, for Nix. There are a few cosmetic problems with the custom GTK icons not being loaded, but from <a href="https://github.com/NixOS/nixpkgs/issues/247#issuecomment-32018293">this comment</a> it looks like this is a known issue? Anyway, I'm not sure how to get GTK to find icons local to nitrogen in the nix store. But other than that nitrogen is working fine.