added a shield to the Valkryrie

- Added new entity type "Attachement".
- The Valkryrie shield is an attachement.
- The shield blocks anything, but cover only the left side of the
Valkyrie due to the small collision box.
- The shield is up only when not moving and not reloading weapon/melee
attack.
- Shield data exported to json.
- Rebalanced Valkryie stats to compensate for new ability.