Adding exercises from the Software Carpentry 4.0 website

Adding exercises from the Software Carpentry 4.0 website

W. Trevor King: I dropped everything from the original da170d3 except
for the exercises/test.markdown addition.  From Jon's initial pull
request [1]:

  Here are a bunch of exercises, markdown-ized, that we used to have
  as part of the website for various topics.

  I wasn't sure where to put them exactly, so I've created a new
  top-level folder for them, and placed them there, flatly. I've named
  things so that the assets for a page of exercises have the same file
  name prefix (e.g. vc.markdown, and vc_planets.txt).

[1]: https://github.com/swcarpentry/boot-camps/pull/34