Game server: ensure that if a 2h weapon is equipped, any item in the the other weapon slot is unequipped. Resolves #185.
Game server: slight tweak to previous commit; only bother returning the item/sending the packet if there is actually an item in that slot (if not, it's pointless).
Game server: fixed the 2h check to allow for 2-handed spear & mace-type weapons.
Game server/skill system: fixed skills simply checking the nation as the be-all-end-all of enemy detection; it now calls Unit::CanAttack() to utilise the more specific logic. Resolves #186.