Fix Inventory Unit Mapping

The orphan-removal flag is on the wrong side of the association and is discarded by doctrine.
This leads to Order::removeInventoryUnit() failing to cascade properly, and makes the cart update fail when reducing the quantity of any item in the cart. Marketing would love to keep this bug, I'm sure ;)
Add hasInventoryUnit method to Order and OrderInterface

This adds consistency between Shipment and InventoryUnit handling.
Specs are updated accordingly.
Merge pull request #353 from Richtermeister/fix-inventory-unit-mapping

Fix Inventory Unit Mapping
Initial new versioning schema