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.
There was actually a discussion on the mailing list a while back about the `filter` module. It's basically a dumping ground for functionality because there is a more colloquial understanding of a filter that doesn't limit it to a convolution operations. The Hough transform is actually more of a filter than the Canny edge detector (which is in the `filter` module).