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;
Yeah that is a bit more difficult because NSCache has no concept of enumerating all objects in the cache. That being said the NSMutableDictionary that also backs FDCache should be a superset of the NSCache. Any object that is in the cache should also be in the dictionary because the dictionary holds onto references to all objects that are still in memory.

With respect to this method would you want it to wait and return back the model if it is being created or if you query this method and the model is still being created you get back nil.