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.

`srcNode` is the node where the interface currently exists, or None for the root ns:

```python
def moveIntfNoRetry( intf, dstNode, srcNode=None, printError=False ):
    """Move interface to node, without retrying.
       intf: string, interface
        dstNode: destination Node
        srcNode: source Node or None (default) for root ns
        printError: if true, print error"""
    intf = str( intf )
    cmd = 'ip link set %s netns %s' % ( intf, dstNode.pid )
    if srcNode:
        cmdOutput = srcNode.cmd( cmd )
    else:
        cmdOutput = quietRun( cmd )
```