Empty buckets replace fluid buckets on use and vice versa (patch contributed by Mgueydan)

Fixes FS #277.
API change: added an optional parameter to cItemGrid:AddItem(), cItemGrid:AddItems(), cInventory:AddItem() and cInventory:AddItems()

git-svn-id: http://mc-server.googlecode.com/svn/trunk@1643 0a769ca7-a7f5-676a-18bf-c427514a06d6