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)


Ah, I see. This is worth repeating above, in the filter function definition. Say that the function is designed to be used with a footprint having `values[0]` as the central element.

I'd prefer to be able to specify a connectivity parameter... Would the following work?

```python
fp = nd.generate_binary_structure(arr.ndim, connectivity)
for d in range(fp.ndim):
    fp.swapaxes(0, d)
    fp[0, ...] = 0
    fp.swapaxes(0, d)
```