Proof of the halting problem by Gernot Lecaks.

Patch provided via email (revision 3 included here).
Partially reviewed by Prof. Slany.
Halting problem update by @wslany.
Halting problem update by Gernot Lecaks.
Yet another halting problem update by Gernot Lecaks.
Replace footnote with paper reference.
Removing LaTeX surrogates.
ja/nein becomes Ja/Nein.
"\emph{berechnen}" becomes "berechnen".

The importance of this verb was pointed out previously in this chapter.
Don't print variables in boldface.
Remove Wikipedia links.
Fix indentation. Prefer ,,Ja`` over \emph{Ja} in text.
Fix phrase. I think this is more understandable.
Rephrasing. I guess this one's more understandable.
Removing another Wikipedia link.
Improve structure of halting problem section.
I already explained this concept in the section of universial TMs.
General review of correctness proof.
Introduce SVG images.
Merge branch 'gl/haltingproblem'