Improve example session layout.
Adds a script to set up NAT-PMP port forwarding, see #6

Note: This script needs the natpmp library installed. This library is
part of the http://miniupnp.free.fr/ software package, and can be
installed on a Debian system via `apt-get install libnatpmp-dev`.

This script also needs the "jit" native script installer, from 
https://github.com/radiospiel/jit.