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)


Actually, why this naming at all?  ``rag_meancolor`` == region adjacence graph mean color.  I'd expect, perhaps,

``from skimage.graph.rag import merge_mean_color``