Sketch out first 3 sections

These sections were originally IPython notebooks that I converted to markdown. This is missing a lot of explanatory text, but that will come later. I'm really not sure whether this is the right process for version control. I suspect, that each build will create new images, but I guess the text diffs are manageable (compared to diffs of ipynb files) so that's a definite plus.

Note that I created a separate file for plot helpers. I don't like having notebooks cluttered with code that isn't relevant to the topic. I plan on discussing the helpers, but just having them in a separate file minimizes distraction.
Actually I think this image makes it a bit confusing, because the result ends up looking exactly like the kernel, which only happens in special cases. How about making it a 7 x 7 image with a 3 x 3 bright patch in the middle? Then the result will be more interesting.