Allow plugins to enchant normally unenchantable items. Adds BUKKIT-1849

It's currently not possible for a plugin to allow enchanting of an
otherwise un-enchantable item using the enchanting table. This commit
causes EnchantItemEvent to be called with an empty list of enchantments,
allowing a plugin to provide it's own set of enchantments when vanilla
Minecraft would not allow any. It also bypasses the unsafe enchantment check
since a plugin should be permitted to apply any enchantments they like, and
vanilla should not generate unsafe enchantments of its own.