flatten in cases where an empty nested array would confuse join.
Add KeyedListLike transformation
Merge pull request #796 from twitter/jco/794

Add KeyedListLike implicit transformation to TypedRichPipeEx
Merge pull request #795 from ljrittle/module-support-with-hdfs-mode-in-scald.rb

flatten in cases where an empty nested array would confuse join.
Merge https://github.com/twitter/scalding into develop