gh-pages: merge bc/gh-pages and add my changes to shell and git

This is not essential. If you don't have time, don't even review it. I can use the version at http://sonoflilit.github.io/2014-07-10-Nottingham/novice/git/, it's perfectly functional.

I merged a recent swcarpentry/bc and then cherry-picked my changes to shell and git lessons (mostly adding concept maps.)