New API for WebSocket support

- API
 - @websocket decorator
 - WSGIApplication#websocketmanager()
- bugfix
 - Even if a connection is cut, it continues remaining
- remove restriction
 - Two or more connections can be accepted

Signed-off-by: Satoshi Kobayashi <satoshi-k@stratosphere.co.jp>
Reviewed-by: YAMADA Hideki <yamada.hideki@po.ntts.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Merge remote-tracking branch 'osrg/master' into ws-rpc-dev

Conflicts:
	ryu/app/simple_switch_websocket_13.py