Extract Stage superclass - create RowTransformationStage

Stage is now just a generic named, executable part of the pipeline. At
some point column dependency management will probably fit in here as
well.
Make clear StageBuilder interface.

Start pulling out logic as to which stage builder to use based on name
& options.
Remove ruby 1.8 ZenTest from Gemfile
Add back ins sensible build_stage method for Pipeline
Stages can now be constructed without being bound to a name.