Add the Nyucore::Metadatum value object spec and implementation for storing the datastream with the metadatum value
Return Nyucore::Metadatum instances instead of plain old Strings from the Nyucore attribute readers
Remove the NyucorePresenter from the NyucoreHelper

The NyucorePresenter was overkill for the simple cases we were concerned with. Nyucore::Metadatum makes it especially useless.
Remove spec and implementation of NyucorePresenter
Leverage Nyucore::Metadatum in the Nyucore form