Fix 403 errors from tornado.access when handling new WebSocket connections

I think an update to python-tornado (I'm on 4.0.2) breaks incoming WebSocket connections in Omnibus.  In Googling around, the common answer is to simply add a check_origin method that returns True.  This mimics the behavior tornado used to have and allows the WebSocket connections gracefully.

Before this commit, these messages would be displayed from omnibusd:

    $ ./manage.py omnibusd
    WARNING:tornado.access:403 GET /ec (127.0.0.1) 0.90ms
    WARNING:tornado.access:403 GET /ec (127.0.0.1) 0.69ms
    WARNING:tornado.access:403 GET /ec (127.0.0.1) 0.51ms
    WARNING:tornado.access:403 GET /ec (127.0.0.1) 0.58ms
    WARNING:tornado.access:403 GET /ec (127.0.0.1) 0.59ms
    WARNING:tornado.access:403 GET /ec (127.0.0.1) 1.05ms
    WARNING:tornado.access:403 GET /ec (127.0.0.1) 1.05ms
    WARNING:tornado.access:403 GET /ec (127.0.0.1) 0.63ms
   ...

Some references:
https://github.com/mopidy/mopidy/issues/788
http://stackoverflow.com/questions/24851207/tornado-403-get-warning-when-opening-websocket
http://stackoverflow.com/questions/25177147/tornadio2-failed-error-during-websocket-handshake-unexpected-response-code