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.
> imshow is redundant. I don't think you need to contrast adjust with vmin and vmax, and I think passing a cmap=cm.gray should be done explicitly.

I agree. Setting the exposure limits was helpful in an earlier incarnation of this notebooks, but it's  no longer necessary and just a distraction.

> Having a black-box function like imshow_rgb_shifted, that isn't particularly intuitive, and that could be done as an exercise, breaks that pattern. In fact, writing imshow_rgb_shifted might be a good exercise!

Well, `imshow_rgb_shifted` is a bit fiddly for an exercise, but I'll make a simpler version into an exercise.