Send return value from @on function in BaseWebsocketHandler to simplify responses

[Josh Humphries]

When you have a handler function which responds to a websocket message using the @on annotation you have to call

yield from self.app.send_async(self.websocket_id, 'something', {'message': 'Hello, world!'})

to send a message back down the websocket.
The return of the function should be taken by the handler and automatically sent back to the websocket (it it is non-null).

[Josh Humphries]

Fixed in b05eef3