[Nix-dev] setting environment values.

[stewart mackenzie]

Tim you need to do something like this:

```
let
      pkgs = import <nixpkgs> {};
      stdenv = pkgs.stdenv;
   in rec {
       erlangEnv = pkgs.myEnvFun {
         name = "erlang";
         buildInputs = with pkgs; [ erlang rebar openssl git binutils
bash emacs which mc meld tig elixir ];
       extraCmds = ''
         export POSTGRES_PASS=password;
         export MAILGUN_KEY=my_mailgun_api_key;
         export MAILGUN_DOMAIN=contact at email.org;
       '';
       };
   }

```