Added Gaia's l10n.js

If you're looking into localizing your app on runtime, search no more.
This library is used in FFOS to localize its apps.

For more information check:

https://hacks.mozilla.org/2013/08/localizing-firefox-os-apps/

http://informationisart.com/26/

(This is the updated version, AKA 2.0)