Simple VNS

Purpose: Provide a temporary testing platform which is compatible with the VNS
         protocol for the PWOSPF project.

How to Use:
  1) Start the local VNS server: python VNS.py

  2) Connect instances of your router - the default topology has four routers:
      sr -s 127.0.0.1 -v rtr1
      ...
      sr -s 127.0.0.1 -v rtr4

  3) You can't inject traffic from outside the network, but you should see your
     PWOSPF traffic going around.  You can also generate traffic with your
     router (e.g., by pinging through your CLI).

Note: This is very new and beta.  I don't doubt there are bugs - let me know if
      you find something wrong ... but it works for me :).  Feedback welcome.


TODO:


- be able to specify the VNS server port (default 12345 is used)


