Finalize the ShipmentItem structure

We want to make the following changes:
1) Stop referencing the purchased_entity_id and purchased_entity_type. We have the order_item_id, that's enough.
2) Add a label, that can be used to describe the item on customs forms
3) Add a declared_value, for the same purpose. Add a Shipment::getTotalDeclaredValue() to match.
4) Add a tariff_code optional property (currently unused).

This makes the ShipmentItem self-sufficient in many cases, including customs forms (needed for international shipments).
We also need a unit test for ShipmentItem.