Add fetchTarball builtin

This function downloads and unpacks the given URL at evaluation
time. This is primarily intended to make it easier to deal with Nix
expressions that have external dependencies. For instance, to fetch
Nixpkgs 14.12:

  with import (fetchTarball https://github.com/NixOS/nixpkgs-channels/archive/nixos-14.12.tar.gz) {};

Or to fetch a specific revision:

  with import (fetchTarball https://github.com/NixOS/nixpkgs/archive/2766a4b44ee6eafae03a042801270c7f6b8ed32a.tar.gz) {};

This patch also adds a ‘fetchurl’ builtin that downloads but doesn't
unpack its argument. Not sure if it's useful though.