updated german translation

Found a typo in the german translation yesterday, fixed it in my installation, found your github, realized you did some more work since the last release. So I used the opportunity to review the entire german translation file, fix some more typos and imprecise translations, check the entries marked as fuzzy and add translations for the new strings.

in case it matters: I used poedit 1.4.6 (could not use the current 1.5.x version because that requires OSX 10.7 and I'm still on Snow Leopard).