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)


Use present imperative: "Do this" instead of "Does this".

Also, it is useful for the reader to know that the resulting node has the id "j". (I had to hunt down this info in the source code.) So, a concise summary might be: "Merge node `i` into node `j`."