Add a utility function "fetchGitHub"

This is a small wrapper around fetchzip. It allows you to say:

  src = fetchGitHub {
    owner = "NixOS";
    repo = "nix";
    rev = "924e19341a5ee488634bc9ce1ea9758ac496afc3"; # or a tag
    sha256 = "1ld1jc26wy0smkg63chvdzsppfw6zy1ykf3mmc50hkx397wcbl09";
  };