Fix error when cloning enchanted ItemStacks.

The existing method for cloning a Bukkit ItemStack fails to correctly copy
the underlying enchantments. Fix this by overriding .clone() in
CraftItemStack, cloning the underlying nms ItemStack correctly.

Fixes BUKKIT-360

src/main/java/org/bukkit/craftbukkit/inventory/CraftItemStack.java

@@ -210,6 +210,11 @@ public class CraftItemStack extends ItemStack {
         return item;
     }
 
+    @Override
+    public CraftItemStack clone() {
+        return new CraftItemStack(this.item.cloneItemStack());
+    }
+
     public static net.minecraft.server.ItemStack createNMSItemStack(ItemStack original) {
         return new CraftItemStack(original).getHandle();
     }