Benp/add total event count

@rocha @brianhw @mulby 

I think it makes sense to get started on review. For now, could you ignore (or at most, skim) the test files? 

The prod code is worth a closer look at this point, and I welcome any feedback.
for better or worse luigi calls `eval()` on the strings provided by hadoop, so these will already be integers. You should be able to do: `yield key, sum(values)`