Lua stuff & other things

Added ItemType.usesSlot to global.lua, now the slot constants actually
have a use.
Changed onChangeHealth & onChangeMana to be more flexible.
Added table.create & Item.hasProperty.
Made Game::combatChangeHealth & combatChangeMana a bit faster.
Fix origin for combat.

Also added combat:setOrigin.
Make onChangeHealth easier to use.
Merge pull request #888 from dalkon/master

Lua changes