Use setuptools to install python files

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

PYTHONPATH mods are no longer required for installation.

examples/multitest.py

@@ -2,7 +2,7 @@
 
 "Run multiple tests on a network."
    
-from mininet import init, TreeNet, pingTestVerbose, iperfTest, Cli
+from mininet.mininet import init, TreeNet, pingTestVerbose, iperfTest, Cli
 
 if __name__ == '__main__':
    init()