a nicer figure for integrate

The figure in the "integrate" definition is about 90% JPEG artifacts, despite
being a prime candidate for some nice vector treatment. I have redone it in
TikZ.

Since TikZ is a massive dependency, I have chosen the conservative variant: an
independent document that is \includegraphics'ed into the main one.
improve the task description of 3/integrate

While I'm at it, overhaul the "integrate" task description (task 3).
Merge pull request #7 from RavuAlHemio/nicer_integrate

nicer integrate