Add in OrdinalScale and CategoryBarRenderer

There is also now a CategoryRender class that descends from Renderer
and has a Orindal x-scale and Quantitive y-scale. If Ordinal Y-scales
are desired a separate class might need to be added later.
Change CategoryBarRenderer to take a width accessor instead of a value
Merge pull request #238 from palantir/add-category-renderer

Add in OrdinalScale and CategoryBarRenderer