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_cpu.py

@@ -15,7 +15,7 @@ cfs	10%	1.29e+09
 """
 
 import unittest
-import pexpect
+from mininet.util import pexpect
 import sys
 
 class testCPU( unittest.TestCase ):