Add querying for instance with identifier

Added the ability to query existing instances of the model for an instance with the specified identifier so I don't have to use duplicate slug => league mappings

 + (instancetype)existingModelWithIdentifier: (id)identifier;
This is probably outside of the scope of this commit but I think it would be worth while switching _existingModelsByClass to FDThreadSafeMutableDictionary and making FDCache thread safe so that you don't have to synchronize on this.

Although synchronizing on this does insure that if a model with the identifier is being created when this method is called it will wait and return back the model.