add bootstrap theme to overview
update navhelp screenshot image for new theme
Merge pull request #118 from isaacdontjelindell/master

Convert exercises in ThinkCSpy to 3-tabbed format
Merge pull request #122 from isaacdontjelindell/overview

Enable Bootstrap theme for overview.
Tweak line height in CodeMirror
Merge branch 'master' of github.com:bnmnetp/runestone