RAG drawing function

![figure_1](https://cloud.githubusercontent.com/assets/3823490/3727712/e797aec6-16a0-11e4-9281-656fe0951a06.png)

![figure_2](https://cloud.githubusercontent.com/assets/3823490/3727719/f2e2a9a2-16a0-11e4-8667-bc06b7f59a9d.png)


I would prefer `None` to be the default. Additionally, depending on what this is passed to, it would be great if any matplotlib colorspec (e.g. 'k' for black, "#0000FF" for blue) would work here.