Allow easily configuring a model class for fake records

This allows easily configuring a model class for fake records.
This is setup for polymorphic associations with fake records to
ask for the model class. This is covered by real records because
the model sets itself on the real record. This never occurs for
fake records so it's necessary to manually configure their model
classes.
Merge pull request #63 from redding/jcr-config-model-class-on-fake

Allow easily configuring a model class for fake records