Normalized Cut on RAGs (WIP)

I will upload the test case real soon, till then I would like to invite comments/suggestions on the code.

All of the below results were computed with the same default values.
## Results
### Images from `data`
![camera_cut](https://cloud.githubusercontent.com/assets/3823490/3666957/d1f002cc-11f9-11e4-99c4-f1da5a27b3ee.png)
![coffee_cut](https://cloud.githubusercontent.com/assets/3823490/3666959/d1f5afb0-11f9-11e4-92bd-66a73cc93823.png)
![coin_cut](https://cloud.githubusercontent.com/assets/3823490/3666958/d1f4855e-11f9-11e4-97c3-447704f301cc.png)

### Custom Images
#### Input
![baby](https://cloud.githubusercontent.com/assets/3823490/3666966/18c50f4e-11fa-11e4-9832-ebc89b1e4cd1.jpg)
#### Output
![baby_cut](https://cloud.githubusercontent.com/assets/3823490/3666969/1e503448-11fa-11e4-94c1-3dfc9a15907c.png)

#### Input
![fruit](https://cloud.githubusercontent.com/assets/3823490/3666973/36895ca6-11fa-11e4-96c2-49655373234b.jpg)
#### Output
![fruit_cut](https://cloud.githubusercontent.com/assets/3823490/3666975/3afbf56e-11fa-11e4-8785-2915cd388748.png)

#### Input
![car](https://cloud.githubusercontent.com/assets/3823490/3666976/400081d8-11fa-11e4-8e39-ba1348d386df.jpg)
#### Output
![car_cut](https://cloud.githubusercontent.com/assets/3823490/3666979/4774681c-11fa-11e4-8d0f-fd02927dc376.png)




They look decent! I'm not sure how they compare to other methods. (Coins is particularly cool for a couple of function calls!) We are not trying to break segmentation accuracy records, but to get good, reliable implementations of reference algorithms. Come to think of it, it would also be valuable to see if you can find a reference ncut implementation to which you could feed the same superpixels, and compare the results.

Another thing I would like to see is the k-means based ncut where you specify the number of segments and it specifies the cut in a single step, rather than recursing down. I'm not sure whether that would fit in this PR or a different one — I leave it up to you. As to the interface, one option would be to provide an `n_segments` kwarg that is set to None by default (recursive 2-way), but that becomes k-means ncut when an integer is given.