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)


I think it will be better if we keep `_add_edge_filter` method agnostic. It can just create the RAGs, and the later methods can define the parameters like 'mean color etc. I have added an if statement later though, instead of try.