Fixing a mistake related to l10n

I noticed the localization files weren't working so i noticed that you use 'meta-box' everywhere, while the load_textdomain is loading the $mofile as "rwmb".

I did a test here and now it works just fine. I also updated the brazilian portuguese .po .mo files, if you are interested.
Merge pull request #520 from TioFelix/patch-1

Fixing a mistake related to l10n