typos
- Fix OccurrenceTraverser to fallback to plone.app.imaging's ImageTraverser
  only for Archetypes based objects. For Dexterity content, image scales are
  accessed only via the @@images view and not by traversing to a image scale
  name. Fixes #78 and fixes #79.
abstact, generalify OccurrenceTraverser stuff
we need objects not accessors here
test OccurrenceTraverser for DX and AT. Fixes #79, Fixes #78