Add static code checking for style and errors

This required a change to logging, which now uses a singleton pattern.

For all future checkins, 'make codecheck' should pass.

mininet/xterm.py

@@ -6,12 +6,12 @@ Utility functions to run an xterm (connected via screen(1)) on each host.
 Requires xterm(1) and GNU screen(1).
 """
 
-import os
 import re
 from subprocess import Popen
 
 from mininet.util import quietRun
 
+
 def makeXterm(node, title):
     '''Run screen on a node, and hook up an xterm.
 
@@ -48,4 +48,4 @@ def makeXterms(nodes, title):
     @param title base title for each
     @return list of created xterm processes
     '''
-    return [makeXterm(node, title) for node in nodes]
+    return [makeXterm(node, title) for node in nodes]