Added hover animations, refactored CTA button markup and CSS, refactored link styles.

Added a 'grow' hover animation from @IanLunn's Hover repository. Also refactored the CTA buttons markup so they were pure links and adjusted CSS accordingly. Finally, I improved the link styling to include :focus pseudo-selectors and made visited links identical to unvisited links.

This branch also includes a change to kramdown, the new default Markdown interpreter.