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'd call this `draw_rag`.
- PEP8: no spaces around `=` for kwargs
- Can we also add a `colormap` argument that takes a matplotlib colormap string and maps the edge colour to that? I know that in many cases this will be less visible, but I still think it could be very useful. One way to make it always visible would be to draw a thick black edge and then a thinner colormapped edge within it. We can experiment with this once the colormap code is in there.
- The low-color and high-color keywords could be replaced by the colormap argument: see [this stackoverflow Q&A](https://stackoverflow.com/questions/9707676/defining-a-discrete-colormap-for-imshow-in-matplotlib) for how to do this. I would do this by defining two custom colormaps within this module or within `draw`: `single_colormap` and `two_color_colormap`.