add nix expression

shell.nix

@@ -0,0 +1,27 @@
+{ pkgs ? import <nixpkgs> {}, mode ? "shell" }:
+
+with pkgs.lib;
+
+let
+
+modes = {
+  shell = "exec fish";
+  test  = "npm test";
+  run   = "exec bin/hubot -a matrix";
+};
+
+in pkgs.stdenv.mkDerivation rec {
+  name = "asjon-dev";
+  source = ".";
+
+  buildInputs = with pkgs; [
+    nodejs nodePackages.coffee-script
+    openssh graphicsmagick
+  ];
+
+  shellHook = environment + getAttr mode modes;
+  environment = ''
+    #set env variables here
+    export AUTO_KILL_ON_UPDATE=1
+  '';
+}