Bug quest

I have added a small bug quest which consists of two tiles: s1e1 and s2e1. This little quest is supposed to be started by going South from e1. This means that the player should not be able to enter the bug quest tiles from any other surrounding tiles except e1. For this purpose, I have added restrictions in the `leave` methods of all surrounding tiles. For the sides that haven't been written yet, I added simple placeholder tiles with the required restriction so that future hackers will know to include it.
I've been testing and fixing it for a couple of days so hopefully there won't be any unintentional bugs. I hope you like it. :)