Fix #1456 (Adds pizzabox check)

Now pizza boxes will check if they already have a pizza inside before adding another one.
Fixes issue #1456 ("Putting a Pizza in a Pizza Fold despawns the older one").

![no_more_wasted_pizza](https://cloud.githubusercontent.com/assets/6355724/4260338/13b5a6c4-3b37-11e4-9119-9d07d6344bab.png)