Localization

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

Line 10 says : if your locale is {locale}, you can find the right locale file here: locales/{locale}.properties (for example locales/fr.properties).

The second line does not seem necessary, but it tells where to find the manifest, where the available locales (and the default one) are listed.

Everything is quite well explained here: https://developer.mozilla.org/fr/Apps/Build/Localization/Localizing_Firefox_OS_Apps