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.
I actually have never heard of this and have no idea how it works! I imagine most people in the crowd will be in the same situation, and will be just as curious as I am to find out! Can you add a bit about how it works? Implementations are best, but a high-level discussion and/or a link to the relevant lines in the skimage source would be preferable.