onChangeHealth/Mana if either return false, do not proceed
Teach Lua about ItemType:getSlotPosition()

For lua programmers without C++ knowledge: This function returns a bitset
which means, you would have to use bit.bitand or whatsoever to test it
against the slot position you want (see items.cpp for more information)

This closes #795
Merge pull request #804 from decltype/lua

Implement ItemType:getSlotPosition()