Summing over axis

First off, thank you for a wonderful, wonderful tool. CyLP has saved me so much time already :+1:

I may well be doing things wrong, but I often find myself wishing I could write constraints like

``` python
s += my_binary_matrix.sum(axis = 1) == 1
```

but it appears that the variables' `sum()` function doesn't support it. Is there another way of expressing sums across a subset of dimensions (that doesn't require writing loops)?