FIX: changes needed to fix permission denied exception on Firefox >= 30

 - https://github.com/emberjs/ember-inspector/issues/147
 - https://blog.mozilla.org/addons/2014/04/10/changes-to-unsafewindow-for-the-add-on-sdk/
Merge pull request #149 from rpl/fix/permission-denied-exception-on-firefox-30

FIX: changes needed to fix permission denied exception on Firefox >= 30