lightdm: Initial expression for lightdm as the display-manager service (using default GTK greeter)

This pull request builds on https://github.com/NixOS/nixpkgs/pull/420 to provide OS integration for using LightDM. Users simply need to add the following to `configuration.nix`:

    services.xserver.displayManager.slim.enable = false;
    services.xserver.displayManager.lightdm.enable = true;

This will use LightDM for the `display-manager` service. The integration should work correctly with the session support that NixOS already uses. My personal configuration is:

    services.xserver.enable = true;
    services.xserver.displayManager.slim.enable = false;
    services.xserver.displayManager.lightdm.enable = true;
    services.xserver.desktopManager.xterm.enable = false;
    services.xserver.windowManager.xmonad.enable = true;
    services.xserver.xkbVariant = "dvorak";

Which gives me a 'none + xmonad' session which does what I'd expect.