Add a method to prepare for optimizing FlatMapKeys
Incorporate review comments
Merge branch 'develop' into transOutput
Remove compiler warning
Merge branch 'develop' into transOutput

Conflicts:
	summingbird-core/src/main/scala/com/twitter/summingbird/Dependants.scala
FlatMapKeys optimization in scalding
Merge pull request #305 from twitter/transOutput

Add a method to prepare for optimizing FlatMapKeys