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 understand your point of view, but since this asks students to add the free parameters, it would require students to add those parameters in *3* different places. I think `**kwargs` is a common-enough idiom to force on students. :) I'll try to remember to clarify this during the tutorial, though.