Also, passing CLI args to NAT constructor

fixes #437

Note that we do still provide a default if you specify
ControlPath=True

Works in a few ways:
 * a single file: --custom foo.py (as before)
 * comma-separated list: --custom foo.py,bar.py
 * multiple custom args: --custom foo.py --custom bar.py

We add a new experimental feature to allow Mininet to run across
a cluster of machines. This is currently implemented via a set
mix-in classes that provide remote nodes that are implemented
via a connection to a remote shell, and remote links which are
tunnels across servers. In this preliminary implementation,
both control and data connections are made via ssh, but this
could change in the future.

A MininetCluster class is provided which allows existing code
to be used with minimal modification - all that is required is
to provide a list of servers to use. A customizable placement
algorithm may also be specified. An experimental CLI subclass
is also provided to make it easier to examine node placement;
status and links commands can also check whether nodes and
tunnels are still running.

Although this is an experimental feature, it does include a
--cluster option to make it convenient to start up a Mininet
simulation over a cluster, and a script to assist with setting
up the prerequisite authentication via ssh key pairs.

The cluster feature is preliminary and missing some obvious
important features, such as parallel startup and multiple tunnel
types, which we hope to add in the future.

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.