Add device model names to topology map

[mikeybeck]

I've made a small edit to the zigbee2dot.py file which adds the device model names to the nodes in the topology map.
It works fine with my network, but I only have a few devices so far.

If you're interested, can you please give it a try and let me know if you'd like me to make a PR?

[Yoda-x]

Hello, sure, please create a PR. I was thinking sometime the same, just showing the nwk ids is not self explaining

[mikeybeck]

Done, thanks!