L20n update

I've cleaned up a bit the code and some stuff that are probably leftovers.

There is a few things that will make this code impossible to land in Gaia at this point:
 - There is no way to select the language depending on the locale of the navigator. Your code translate everything in french and that's all.

 - j20n sucks, really and you should never use that to fill your resources['system'] object - declare it inside l20n.js and kill the sys.j20n file.

  I've listen your opinion about json last time and I now agree that we need a dedicated format for l10n but... Gaia won't live with an |eval|... Let's create a parser for your LOL format (the file is unused btw) and let's start again from that.

 - l10n-id, l10n-path, l10n-args should be prefixed by 'data-'. This is the standard in HTML and it let developers manipulate those directly from Javascriipt (for example. node.dataset.l10n-id = 'foo'|


My path does not addressed those points since you probably have some ideas/opinions about it.