Add WAMP to the list of allowed websocket subprotocols.

This request allows libraries like https://github.com/blopker/WAMPlay to use AutobahnJS for websocket RPC/PubSub.

Originally the subprotocol list was `null`. This tells Netty to reject any header with the "Sec-WebSocket-Protocol" field set. Setting the string to `"wamp"` allows AutobahnJS to connect to Play without using ugly workarounds. To add further protocols (though I'm not aware of others) just set the string to `"wamp,otherprotocol"`.