POC Mermaid diagrams

:warning: Do not merge this, as the exercise is for practice and demo purposes only.

This was a simple proof of concept to demonstrate the diagrams built with [Mermaid](https://github.com/knsv/mermaid) and Markdown over fixed-content SVG content.

If @github/training-teachers sees the implementation worthwhile, a full move to Mermaid might happen in 1Q 2015. Otherwise this PR and branch will be :fire: after a quick discussion.

![screen shot 2014-12-05 at 1 03 46 pm](https://cloud.githubusercontent.com/assets/352082/5322090/4ea4f86e-7c7f-11e4-9668-45be6f55a5a2.png)