Add AssociationProxy support

This fixes issue #148, adding support for [SQLAlchemy's AssociationProxy](http://docs.sqlalchemy.org/en/rel_0_8/orm/extensions/associationproxy.html) pattern. We are using this pattern in several places where we need ordered many-to-many relationships, and this allows those relationships to work transparently in the Flask-Restless API just like they do from Python.

I wasn't quite sure where to put the tests, or how to structure them, but the current tests do cover the whole AssociationProxy functionality quite completely.