Region Adjacency Graph

## Example Images
![figure_1](https://cloud.githubusercontent.com/assets/3823490/3291718/5ce9b952-f587-11e3-8c29-e2aace6d7794.png)
![figure_2](https://cloud.githubusercontent.com/assets/3823490/3291723/7074b134-f587-11e3-8c73-d28222891a21.png)


My preference is to avoid underscores where they are not necessary. So I would replace `pixel_count` with `pixel count`, `pixel-count`, or just `npixels`. Same below: `total color` or just `color`. @stefanv Do you have an opinion on this?