[Bleeding] Correctly enchant books. Fixes BUKKIT-5302

Books can now recieve more than one enchantment.  As such, breaking out of
the loop after only 1 enchantment is applied is not the correct behavior.

This commit also reworks some of the logic surrounding the application of
enchantments to the item. By checking if the event doesn't add any
enchantments rather than if the original enchantments list is empty, the
application code is only reached if enchantments are applied, rendering
the "applied" boolean no longer necessary. The ItemStack's Item should only
be set once, so it is now set outside of the loop based upon whether or not
"flag" is true (with "flag" being whether or not the ItemStack's Item is a
book).