Update docs.

 * switch from Select2 v3.4.2 to 3.5.1 for the docs homepage
 * load html5shiv.js from maxcdn instead of cloudflare, update from v3.6.2 to 3.7.2
 * remove credits/link to contributors from footer
 * improve Sass organization
 * remove .select2-demos
 * links to demo pages are now handled via a Select2 dropdown
 * make direct use of Bootstraps' variables in gh-pages CSS
 * improve docs homepage UI