updated German language file de_DE.po

Bonjour Nikeo, greetings to Nice,

looked into your theme today and found some parts not being translated into German (which comes, I believe from hard work, adding new features). So I thought, I could give you a hand with a pull request for a new translation.

So far, the translation has used formal German ("Sie") while I see most WordPress users going for the informal version ("du") which is why I changed it accordingly. (The formal language files should be de_DE_Sie.mo/po then, just in case someone misses it and wants to contribute himself.)

I also found a little typo in inc/admin/class-fire-admin_init.php ("do" instead of "to"). Little things matter. :smile: 

À bientôt,
pixolin