Leaves UPnP forward when stopped

How to reproduce:
1. start with ```--tor disable```
2. stop gracefully with SIGINT
3. observe that the router still has UPnP port forwarded:
> ZeroNet | Yes | 15441 | 15441 | 192.168.1.10 | TCP

It should delete UPnP forwards when stopped gracefully.

As a good example, BitMessage handles UPnP very well: https://github.com/Bitmessage/PyBitmessage
I believe they have their own UPnP python implementation.