Split AssociationProxy into Association class (and subclasses)
which manage the association and CollectionProxy which is *only* a
proxy. Singular associations no longer have a proxy.

This mirrors
https://github.com/rails/rails/commit/1644663ba7f678d178deab2bf1629dc05626f85b

and brings the ActiveFedora associations closer to ActiveRecord 3.1
Merge pull request #208 from projecthydra/association_builders

Split AssociationProxy into Association class (and subclasses)