GFM Extensions

This branch includes further improvements to PR #93. They are about GitHub flavored Markdown.

Inner highlighting of fenced code blocks implemented. Credit goes to https://github.com/aziz/knockdown/ Screenshot:

![ekran grnts 2013 10 27 21_52_51](https://f.cloud.github.com/assets/844008/1416430/897acdc8-3f50-11e3-9da2-08210b181fee.png)

Fixes #12, #90, #91, #92