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 was thinking that we'd build up a whole bunch of merging strategies, so do you think those would be better named ``rag_*`` or ``merge_*``?  The module name can also be a hint, hence the reference to ``from skimage.graph.rag import merge_mean_color``.

I'm not sure about the argument for the output type, because we don't call our other functions "image_histogram_equalize" or "image_threshold".