Add nixos backend for system "updates"

[teto]

I've switched to nixos and maintain a local copy of nixpkgs (i.e. the equivalent of deb repostiories aka packages list).
Ideally I would like to see if my local repository is on par with the official one https://github.com/nixos/nixpkgs. As such it basically consists in comparing different git repos revisions.

I am a bit short of time but eventually will try to prepare a PR, until then I use http://howoldis.herokuapp.com/