Add German translation

It’s taken me a while, but I’m finally done with my German translation of the walkthrough! (Updated with the latest code changes on `1.1`.)

To review the translation (pretty please! I’m not *that* confident about it), I recommend looking at ee145b09f8a963e7bd9e0ecc4dbce1d04a4b5b22 in split diff mode. (In case it’s GCed later, I overwrote the english files with the german ones so you get a nice diff.)

5ca70fd is, strictly speaking, not related… I just did it because seeing the backup files in my `git st` was very annoying. (I wrote this mostly during train travels, on a netbook not really powerful enough for a better text editor than `emacs`.) If you don’t want it, I can remove it.
// Die Summe aller Quadratzahlen von 0 bis 10.
(only 0..10, and probably remove "!" because of not being a factorial)