Grouping configurable through catalog entry

[MichaelDrogalis]

From the mailing list:

"Also, it'd be really nice to specify a group-by operation on the input queue of each function, because then you could make things like wordcount really easy--you'd be able to say "send all instances of the same word to the the same downstream task", which would enable workflows that require an implicit sort/shuffle step."

https://groups.google.com/forum/#!topic/onyx-user/xniQcgCPEn8

[MichaelDrogalis]

Completed and merged into master. Will release in 0.4.0.