Removed hash-bang in module files which aren't executed directly,

with the exception of topo.py, which I haven't touched.

mininet/cli.py

-#!/usr/bin/python
-
 """
 A simple command-line interface for Mininet.
 

mininet/net.py

-#!/usr/bin/python
 """
 
     Mininet: A simple networking testbed for OpenFlow!

mininet/node.py

-#!/usr/bin/env python
-
 """
 Node objects for Mininet.
 

mininet/util.py

-#!/usr/bin/env python
-
 "Utility functions for Mininet."
 
 from time import sleep

mininet/xterm.py

-#!/usr/bin/env python
-
 """
 XTerm creation and cleanup.
 Utility functions to run an xterm (connected via screen(1)) on each host.