bug fix (rails workaround) for STI - propagate associations to subclasses.

Rails doesn’t always propagate associations from a base class to subclasses, depending on the load order.  See http://stackoverflow.com/questions/18774605/rails-3-2-3-sti-ar-subclasses-not-inheriting-associations-reflections

fix is to explicitly define the association on the base class and all descendants.