Implement StreamSummary data structure for finding frequent/top-k items
Some changes to StreamSummary as suggested by Oscar in pullreq comments
Rename StreamSummary to less generic SpaceSaver
address oscar's comments about case classes, iterator undefined behavior, secondary constructors, and beefing up unit test to include SSMany instances for semigroup laws
add implicit semigroup for SpaceSaver and use it in unit test
Merge pull request #259 from tresata/feature/streamsummary

Implement StreamSummary data structure for finding frequent/top-k items