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)


`labels` and `label`  ( which is used later ) might create an ambiguity. I think `label_image` is appropriate because it emphasizes that it's an image and not anything else ( like a list of labels, or one single label )