add ability to do avg (multi-part-aggregations) in ClojurePlatform
logic for ParallelBuffer (although limit-rank still doesn't work)
fix logic for limit-rank
add mapcat functionality
throw exception on airity mismatch
remove zipmap in favor of to-tuple
Merge pull request #3 from bhiles/bsh-more-clj-engine-capabilities

Logic for mapcat and ParallelBuffer