Hmm. That's not what I expected Left/RightJoin to do with multiple pipes. Either way it's confusing.

I want to keep `co-group*` close to the raw API and add wrappers to support the common cases, like scalding's joinWithSmaller/Larger and leftJoinWithSmaller/Larger, as well, as join-many that would take a pipe as you describe above. I'm thinking removing `:left` and `:right` keywords for join type, and adding `:exists` using cascalog joiner. Thoughts?

Also, do you expect the logic engine to generate a join map?