Track the model class on the record class

This is to setup handling polymorphic associations. For these, it
will be necessary to determine a model class via a record class.
When a polymorphic association is called, it will fetch the record,
and use the record's class to determine the appropriate model class
to instantiate. This adds logic to store the model class when a
model has it's record class configured. This way the 2 classes
stay synced up and know of each other.
Merge pull request #57 from redding/jcr-track-model-class-on-records

Track the model class on the record class