CSS: minor tweaks.
Move normalize.min.css in _includes and update it to v3.0.1.

Also, concatenate the CSS files on build.
Use more variables.
Remove unneeded end tag.
Use html5shiv, selectivizr.js and respond.js from OSSCDN.

This actually fixes page display with IE < 9.
Losslessly compress img/dependency-graph.png.

Before: 155889 bytes
After:  105188 bytes
Update Jekyll config.

* use kramdown since it will become the default soon
* explicitly disable Pygments since it's not used
* set encoding to UTF-8