First crack at Python 3 compatibility

Mostly we just use mininet.util.pexpect so that we can use
unicode strings without changing calls to pexpect.spawn()

examples/test/test_hwintf.py

@@ -7,7 +7,7 @@ Test for hwintf.py
 import unittest
 import re
 
-import pexpect
+from mininet.util import pexpect
 
 from mininet.log import setLogLevel
 from mininet.node import Node