Streamlined the mapreduce API a bit and made Combiner and Reducer more consistent in design
Merge pull request #2564 from noctarius/remove-odd-mr-api-thinggys

Streamlined the mapreduce API a bit and made Combiner and Reducer more c...
Added first aggregation classes
Trying to get first aggregation running
Added emit to aggregation mapper
Added emit to aggregation mapper
Added most basic aggregations
Added count aggregation
Made generics on public interface easier for Java 6 / 7
Added way more aggregation tests
Added unittest for count aggregation, fixed this aggregation
Added distinct values aggregation
Made internal aggregation classes IdentifiedDataSerializable
Made KeyPredicateSupplier and PredicateSupplier IdentifiedDataSerializable
Changed internal mapreduce operations to cleaned API style