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 actually not a fan of the `skdemo` approach. In particular:

- `imshow` is redundant. I don't think you need to contrast adjust with vmin and vmax, and I think passing a cmap=cm.gray should be done explicitly.
- You can do it explicitly in a single call in the notebook as: `import functools; imshow = functools.partial(imshow, cmap=cm.gray)`
- I think the whole point of this tutorial is to demystify images and image processing. Having a black-box function like `imshow_rgb_shifted`, that isn't particularly intuitive, and that could be done as an exercise, breaks that pattern. I would at least add a `%psource <function_name>` for each so that people can see what's going on.
- in fact, *writing* `imshow_rgb_shifted` might be a good exercise!