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)


Temporarily, a (float, float) -> float function might be sufficient. But in the long run, more complex functions might be required. For example, if we want to take the weighted average of w1 and w2 based on the sizes of the two edges, we need not just w1 and w2 but also those sizes. Generally, the function could take as input g and the two edges, and return a float, for example.