Includes automatic NAT feature (mn --nat) and addNAT convenience method for topologies
fixes #111

IVS is an open source virtual switch available for download at
https://github.com/floodlight/ivs. It uses the openvswitch
kernel module.

Fixes #130

Becomes consistent with the OpenFlow walkthrough & MN walkthrough web pages.

I wasn't sure this was worth fixing at first, but it does look more readable
now.

Also add check to see if another controller is running -
eventually we should really detect errors from starting the
controller!!

Any code in mn is not usable by other Python code.

Hence, move this code into util, so other scripts can use it.

Now customConstructor, because it general to both links and nodes.