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)


The `min_edge` function provides desirable behavior in case of hierarchical merging by thresholding edge weights. The `max_edge` function on the other hand is purely for demonstration purposes

I will copy/paste the same function in both places though