Option to Prune Objects

This allows for an `NSOperation` or direct `NSManagedObject` API to be used to import data and delete any objects held locally that are not present in the data about to be imported. This is good for API's that don't provided an `updated-since` to parameter when `GET`ing data. In these cases data can be imported and any local objects that have been deleted from the server will be deleted locally because they are not present in the full `GET` data about to be imported.