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'll elaborate a bit more: the point of the tutorial is to learn to use numpy, scipy, and scikit-image to do some image analysis. If we introduce some fourth library, `skdemo`, that's not even a real library, then when they come out of it, they won't have much to show for it!