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)


@ahojnnes the comment follows the block, which I agree is backwards. (... right?) But @vighneshbirodkar it might help if you show the 2D example: [[0, 0, 0], [0, 1, 1], [0, 1, 0]] for connectivity=1, and [[0, 0, 0], [0, 1, 1], [0, 1, 1]] for connectivity=2.