load localize.js asynchronously so it isn't page-blocking

My google PageSpeed result is telling me that I should remove render-blocking scripts, and lists https://cdn.localizejs.com/localize.js as one.

With the help of the https://www.drupal.org/project/async_js contrib module, we should be able to add support for async by doing something like this...

-    drupal_add_js('https://cdn.localizejs.com/localize.js', 'external');
+    drupal_add_js('https://cdn.localizejs.com/localize.js', array('external', 'async_js' => TRUE));