Temporarily fix for firefox 30+ background script

This fixes https://github.com/trigger-corp/browser-extensions/issues/36, until a long term solution is implemented (https://blog.mozilla.org/addons/2014/04/10/changes-to-unsafewindow-for-the-add-on-sdk/)
Merge pull request #41 from reinaldo13/firefox-background-script-issue

Temporarily fix for firefox 30+ background script