French translation update

This pull request brings the French translation up to date with the latest string changes (as of 30/03/2013), and also fixes some remaining typos.

On the subject of translation: what would be the best way for me to get notified before a new version is released, so that I can see whether new strings need to be translated? I browse  this repository regularly, but I kind of missed the 0.35 train translation-wise.  : )

Regards,
Alexis