add STP application

This application provides the simple example of spanning tree.

"stplib" module controls exchange of BPDU packets
 and each bridge's port status(relay or block of frames).
The module sends "EventTopologyChange" event
 when the topology(the route which relays frames) changed.

"simple_switch_stp" module is a variation of "simple_switch".
 when the module received "EventTopologyChange" event,
 the module resets flow entries.

to run:
 $ ryu-manager ryu/app/simple_switch_stp.py

Signed-off-by: WATANABE Fumitaka <watanabe.fumitaka@nttcom.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>