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)


@jni , the edge with weight 7 is merged. Between 5 and 15 I suggest we keep 5 ( by default ). Both yours and mine interface allow the user to make the decision, although I had also thought of the 3 vertex API initially before deciding in favor of passing 2 edges.

Or we can do nothing and force the user to give a callback.