Make some classes, traits, etc less visible.

In an effort to make the docs less noisy, this hides some classes/traits
that were previously public, but probably shouldn't have been.
nest the stacks of low priority traits inside objects

fix RowExtractorLow1 extending RowExtractorLow0
Merge pull request #30 from pellucidanalytics/topic/visibility-clean-up

Make some classes, traits, etc less visible.