No longer using net.minecraft.server

Since bukkit's ItemStack has proper serialization/deserialization, we no
longer need to use net.minecraft.server's CraftItemStack to get
information of books and junk. Old chests, which haven't been used since
bukkit version X, will not lose some information, like the text in
written books.
Fixed issue https://github.com/Mtihc/TreasureChest/issues/36

Fixed the impossible bug, thanks to @DerFlash