pull ExprOp out of pipeline.scala

Conflicts:
	src/main/scala/slamdata/engine/physical/mongodb/pipeline.scala
break out all merging-related code to merge.scala
(not including the merge() methods, yet)
pull out all merging-related code to merge.scala
Merge pull request #100 from mossprescott/refactor-pipeline-2

#51: pulled out all merging-related code to merge.scala