Localization

Support for localization. Fixes #1.
Includes commits for sharing (pull request #20) since I'm tired of fighting with git ;)

This is l10n.js from the Gaia source, under apache 2.0 license.
Is there something we need to do to fulfill the license? Maybe  a line in the readme could be nice.
https://github.com/mozilla-b2g/gaia/blob/master/shared/js/l10n.js