accept cross-origin websocket connections by default

Tornado 4 introduced 403 status code in case of cross-origin
websocket connections. SockJS must work with cross-origin websockets
so `websocket_allow_origin` option was added here. By default its
value is "*" - i.e. accept all websocket connections. But it can also
be a list of allowed origins
Merge pull request #64 from FZambia/websocket_cross_origin

accept cross-origin websocket connections by default