Adds SummingIterator
Merge branch 'develop' into feature/cachekey-sum
move files to fit the new project structure
Adds cache-based StatefulSummer and SummingIterator
make a constructor param private on summingcache
Cleans up SummingIterator
Addresses Sam's comments
Merge pull request #112 from twitter/feature/cachekey-sum

Caching paritial sums