Make the class name an option for fake record associations

This makes the class name optional for fake record associations.
This sets up defining fake polymorphic associations, as they do
not specify a class name option. Like ActiveRecord, these
associations will look at the record's data to determine the
class name.
Merge pull request #64 from redding/jcr-make-class-name-optional-fake-associations

Make the class name an option for fake record associations