Item code improvements

I made a lot of under-the-hood improvements to the inventory handling code on the server. I made it more robust by throwing exceptions when an operation isn't possible instead of just returning null or false. I also added a crude transaction system in order to be able to easily roll-back inventories should an error occur during trade completion. It's crude in the sense that it creates a complete backup copy of the inventory content, and in case of a rollback it empties the content and restores it from the copy. I could do this more elegantly when I would invest some time, but the code isn't performance-critical and I want something simple which doesn't leave much room for bugs.

Any glitches in the inventory code could cause serious, permanent harm in the game by duplicating or destroying valuable items (and I already found and fixed bugs which caused both), so the quality of the inventory handling code is very important.