Allow .yml file extension for config files (fixes #645)
Make delta iterations use Keys.FieldPositionKeys
Speed up execution by having an early heartbeat
Add support for serializing null strings.
Fix bug in binary copying of long strings.
Add tests for string serialization.
Add base utility for serializer tests.
Add string array serializer with test.
Added Grouping unit test, extended Reduce integration test, added JavaDocs to ReduceFunction