Updated the French locale to sound more natural

This is the first time I do a pull request.

I've done my best, but there are some places which can't be translated like "Published" or not well like "Revisions for" which varies in French depending on the word following.

Translations of label, slug and identifier could certainly be improved.
Merge pull request #292 from kiplantt/master

Updated the French locale to make it sound more natural
Tweaked some French translations
Added translations of checkboxes is_published and is_mirrored
Removed locale=en when calling number_to_human_size to let rails-i18n do the translation
Merge pull request #293 from davidperrenoud/master

Polishing translations
adjusting small things
update gitignore
Merge remote-tracking branch 'upstream/master'
allow the use of extra headings in editor