Added German translation

Thanks for the nice app, great job :)

So here's the German translation. 

Notes:

* There's a typo in the English version: "Change bacground", should be "Change background"
* I am not sure what you are planning to do with the "... translation by ..." messages in the About dialog. So i just added an additional message for the German translation and also translated the one for the Swedish translation, but I did not add another Label for it. Best idea might be to just display the translator for the currently active locale. Otherwise translators will end up meta-translating all other translators.

Best regards,
Jens :)