Cluster Support Prototype

This is the beginning of providing some basic cluster support in Mininet proper. It is currently implemented as a few scripts in examples/ as well as a utility script to help with setting up the authentication via ssh. It's certainly a work in progress and has many limitations, but the ultimate goal is to provide seamless operation over a cluster with minimal changes to the Mininet API, CLI, and command line usage.

It shouldn't undo my changes, I only added mac into the constructor.