Make providing a class name optional for base association

This modifies the model associations base to make the associated
class name optional. This will allow adding polymorphic associations,
that do not have a static class name, but still allow them to build
on the current association logic. The one-to-one and one-to-many
associations still require passing an associated class name as they
did before.
Merge pull request #58 from redding/jcr-make-class-name-optional

Make providing a class name optional for base association