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)


Maybe rename this as "labels", "labeled_image", or "labeled_field", as above.

@stefanv @ahojnnes @JDWarner @sciunto I think we should agree on a convention in scikit-image for both input images (currently peppered as "im", "img", and "image" in various parts of the code), labeled images, and boundary images. Thoughts? My preference is "label_field" > "label_image" > "labeled_image" > "labels".