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 guess we can allow RAG drawing for black and white images, using gray2rgb?

And **actually**, this just occurred to me: in many applications, the image will only be for illustration, and what one really wants is to see the fine detail of the RAG (e.g. with the cubehelix or hot colormaps). For those applications, even converting a *color* image *to* grayscale would be advantageous!

For this, I would add an option, `desaturate_image=False`, which would call rgb2gray on the image when set to `True`.

If you place the optional `rgb2gray` call after the `gray2rgb` call, it should work seamlessly with black and white images.

Does this make sense?