nixpkgs/morph version handling improvements

Created by: exarkun

Fixes #27 (closed)

This doesn't really remove the NIX_PATH impurity. Instead it encodes it in a shell.nix alongside a morph installation. I think this might be good enough since you can't do anything with this until you get morph installed and this shell.nix is going to be the easiest (and documented) way to get morph.