Use Jekyll's default Markdown parser and highligher

With the current version of Jekyll (2.4.0), this changes the Markdown parser to Kramdown.

If you're curious about the differences in the generated site, [here's a diff](https://gist.github.com/nicolasmccurdy/44dac380596785571d2d). Note that most of the changes involve smart quotes and other typography features.

Fixes #56.

Advantages
---
- default markdown parser for Jekyll, so it follows the conventions of other Jekyll projects
- smart quotes and other typography features
- support for extra markdown features like tables