Allow to work in non-browser env

‘window’ is specific to browser environments.  The convnetjs variable
has already been made into a global variable at this point, so the
assignment is redundant.  This allows it to work in non-browser
environments other than node.

src/convnet_export.js

@@ -1,7 +1,8 @@
 (function(lib) {
   "use strict";
   if (typeof module === "undefined" || typeof module.exports === "undefined") {
-    window.convnetjs = lib; // in ordinary browser attach library to window
+    // By declaring convnetjs globally, we have already made it available.
+    // Nothing to do here.
   } else {
     module.exports = lib; // in nodejs
   }