To create a method to persist and re-instantiate Mantle MTLModel objects to NSUbiquitousKeyValueStore (iCloud key-value store).
NSUbiquitousKeyValueStore can only store values of class NSNumber, NSString, NSDate, NSData, NSArray, and NSDictionary ("plist types"). The MTLNSUbiquitousKeyValueStore category on MTLModel has been designed to create an NSDictionary representation of a model object in a deeply recursive way, making it suitable for storage in an NSUbiquitousKeyValueStore.