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)


this is not very clearly written.  What does "it" refer to--both nodes, the merged node?  If so, write it explicitly: "the edge weight of all edges incident on either node" or "the edge weight of all edges incident on the resulting node"