Update to nix 1.7

This PR updates to nix 1.7, auto-updates to the latest nix, clarifies which XCode version needs to be used with which OSX version, and includes an uninstall.sh script.

It works great on 10.9, on 10.10 nixops is complaining about some linux/darwin mismatch (see https://gist.github.com/rened/6d4792e9be71fceb0cdf, perhaps you have an idea what is wrong), still looking into this.