Added support for villager trading

Hi,

Someone [mentioned](http://dev.bukkit.org/server-mods/itemrenamer/#c72) that villager trades are not covered by ItemRenamer, so I figured I could give it a go and implement it myself. How hard could it be? :P

Luckily, I could pretty much copy the code straight from MCP, and Minecraft itself can handle reading and writing item stacks in the byte array itself (StreamTools).

I've traded a few items now, and it seems to be working fine. No client crashes yet. :)

aadnk.