Adds keypath mapping support

[dzenbot]

By optionally returning +keyPathForRemoteKey: in any NSManagedObject subclass, it checks for a keypath related to the remote key, to be able to call -valueForKeyPath: instead of -objectForKey:

This allows keypaths mappings, for objects that don't need to be nested in CoreData but come nested from the remote payload.

It may still need:

• caching support (to avoid having to +keyPathForRemoteKey: at every iteration)
• skip the discarding log, when the property in class is not found
• refactor refactor refactor