Massively improved shop/menu system

Renamed rp_itembox to rp_ammobox. Cleaned up and improved clientside
shipment entity code. Rewrote spawned_shipment. Renamed spawned_weapon
to spawned_item. Moved sh_weapons and sh_drugs into rp_modules/shop/.
Switched gun dealer/black market dealer menus to new system. Renamed
some act commands (#71). Added sh_shop, which handles shops
automatically using uids (fixes #83). Changed local function changeteam
to RP:ChangeTeam. Changed sv_shipments to use new system. Unified spawn
functions into RP:SpawnItem and RP:SpawnShipment. Removed !buyammo,
!buy, !setprice and !buyshipment. Fixed !job. Added RP:GetConstantN.
Modified includes system so things can be loaded in a specific order
(e.g. loading shop system before shop modules).