Creating slides to introduce lessons

This pull request is starting to create one short slide deck for each subject we teach, along with a short introduction to the bootcamp as a whole, and a two-minute pitch people can use to attract backing.  Before merging, we need to:

1.  Fix the CSS problems (there are plenty of these).
2.  Clean up the wording of the two decks currently in the PR.
3.  Add slide decks for the shell, Git, Python, R, and SQL.

We can use [this deck](http://www.slideshare.net/flxlex/why-of-version-control) by @lexnederbragt as a starting point for the Git deck.