Use a properties file so that we can localize strings

[jkakar]

I started to do this but ran into some issues. I've decided to forget
about it for now, in order to keep moving forward, but in the future
we should do this so that we can translate the extension for use in
different languages.

This page describes the steps you need to undertake to localize the
extension:

https://developer.mozilla.org/en/Localizing_an_extension