Add PrepareItemRepairEvent hook in to anvil. Fixes BUKKIT-2850

Triggers Anvil event created in the following pull request to the bukkit API:

https://github.com/Bukkit/Bukkit/pull/770

Depends on that pull request being in the linked Bukkit JAR.

I've observed that the client UI doesn't give the correct number of levels if you change it on the event, and displays the wrong item type if only renaming. This still provides the option to restrict certain anvil uses.