Group aggregate #1546

@taldcroft please have a look at the changes. Also here is a comparison of np.sum and np.add.reduceat.
![comparison](https://cloud.githubusercontent.com/assets/5782112/3280724/346b350a-f479-11e3-9f7f-d364b806d2b4.png)

Make a link for reduceat like:
```
`numpy.ufunc.reduceat`
```