Use setuptools to install python files

Now, to reference mininet files, use 'import mininet.mininet'.

PYTHONPATH mods are no longer required for installation.

examples/sshd.py

@@ -11,7 +11,7 @@ unchanged on mininet and hardware.
 """
 
 import sys ; readline = sys.stdin.readline
-from mininet import init, Node, createLink, TreeNet, Cli
+from mininet.mininet import init, Node, createLink, TreeNet, Cli
 
 def nets( hosts ):
    "Return list of networks (/24) for hosts."