Remove "from __future__ import print"

This was well-intentioned, but it causes more trouble
than it's worth.

Fixes #652

examples/nat.py

@@ -4,7 +4,6 @@
 Example to create a Mininet topology and connect it to the internet via NAT
 """
 
-from __future__ import print_function
 
 from mininet.cli import CLI
 from mininet.log import lg