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.
Yeah it won't reduce the number of output files, it will just run a reduce pass on the data computed by each mapper so instead of transmitting a bunch of "date, 1" strings it will actually pre-aggregate by date before sending to the reducer which will perform the global sum. The output should be identical, it is simply a performance optimization.