Issue #390: Remove FactMutator and deal with BytesWritable directly
Merge pull request #396 from ambiata/topic/remove-fact-mutator

Remove FactMutator and deal with BytesWritable directly
Fixed KeyValue getOrElse
Introduce KeyValue getOrNull
Improve CountByReducerSpec
Introduce count_by with a secondary field
Remove unnecessary string interpolations from Expression
Merge pull request #394 from ambiata/topic/count-by-secondary

Introduce count_by with a secondary field
Issue #387: Only run squash once for multiple outputs