Create l2_switching_v5.py

The earlier version v4 works ok for a single switched topology. But the code doesn't initialise multiple instances of switches for multiple switch connections/sessions. Thus , multiple instances of macaddr table are not created and a singel instance gets re-written each time. 
In this version, I tried to call multiple switch instances, one for each new connection established. That justifies why the sample code forwarding/l2_learning was written they way it is!
detailed description : http://pox.brownbytes.com/?p=60