Added French translation

Translation mainly rely on YouTube's translation.
Merge pull request #1 from chrisrobins/master

Merge from original
Merge pull request #2 from GeoffreyFrogeye/master

Merge from original
Updated french translation for v0.5.1

Placed directly in _locales.
I made some alterations from the original, e.g. "vidéo" (video) is replaced by "lecteur" (player) because it sounds way less weird.
I've also not seen "toggle_disabled" and "size_checked" & "size_unchecked" description in the extension, so verbs conjugation may seems out of context.
Deleted /_locales/partial/fr.json

As /_locales/fr/messages.json is no longer partial, this file can be deleted.
Updated french translation mistake

Finally found the context of "toggle_disabled" so I corrected the translation. Good thing that I had to listen to a music that I didn't have in my library.
Merge pull request #36 from GeoffreyFrogeye/frenchTranslation

Updated french translation for v0.5.1