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.
Oh, it's just a uint8 issue. I pose the question a few lines down.

I'm really not sure what to do about answers to questions and exercises: I don't want to include them in the notebooks that students have during the class, but I think there should be a way to get the answers (at least after the class). I'm not sure whether I should just update the notebooks after the class or have the answers on a page somewhere else.