handle Read from non-trivial path in workflow builder
Convert WorkflowPhase from PhaseE to Phase.

So we can collect errors whenever a node can’t be created, but only report them if we don’t have a Workflow at the end.
Convert JsExprPhase from PhaseE.
Convert SelectorPhase from PhaseE.
Convert FieldPhase from PhaseE.
Add a test for filter that requires JS.
Merge pull request #256 from mossprescott/paths-quick-fix

handle Read from non-trivial path in workflow builder
Add test for mixing JS and non-JS filter.
Merge remote-tracking branch 'upstream/master' into joins

Conflicts:
	src/main/scala/slamdata/engine/physical/mongodb/planner.scala
Merge pull request #257 from sellout/joins

arbitrary JS in filters
Merge branch 'master' into minor
handle MapReduceTask in RenderTree
also a RenderTree[Js] instance