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.
Yeah, you're right: It's a bit contrived. I'm going to make the detection a bit less convenient in RGB space: Detect the cats eyes. Also, this makes more sense after thresholding, so I'm going to move the whole section. No worries about not know what to do; this is definitely one I plan to give hints on, but only after they spend a couple of minutes thinking over the problem. For most of the exercises, there's a bit more explanation and hinting that I don't want to put in the notebooks so students aren't tempted to look ahead.