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'm still not sure how hysteresis works:

1) Does the thresholding happen on the "non-maximally supressed" image, or on the original gradient magnitude image? (The image shown below appears to be the latter).
2) Do the low threshold pixels need to be *adjacent* to the high threshold ones, or just connected by a "bridge" of low threshold ones?
3) Does the connection happen over maximally-suppressed pixels, or any pixels in the original image?
4) What's the actual algorithm achieved for finding these?