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.
> //just to whet your appetite, the sum of
//all the square numbers from 0 to 100!
//the ^ operator performs exponentiation

I assume the 0 to 100 refers to the *result*, and the `!` is present in the english version as well.

It *should* probably be
> the sum of all the square numbers from 1 to 10!

But we need some punctuation after the `10` (because there’s another sentence behind it), and I think for a “to whet your appetite” sentence, an exclamation mark is more appropriate.

Anyways, that should be done in a different pull request, as the “error” is also present in the other language versions.