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_multilink.py

@@ -6,7 +6,7 @@ validates mininet interfaces against systems interfaces
 '''
 
 import unittest
-import pexpect
+from mininet.util import pexpect
 
 class testMultiLink( unittest.TestCase ):