Support legacy networks (WebSocket alternatives)

[davidje13]

Legacy networks do not support websockets, or close them quickly. To reach a wide audience, alternatives should be used in these cases.

• https://github.com/socketio/engine.io
• https://pusher.com/docs/channels/using_channels/websocket-fallbacks

We can assume modern browsers, so there is no need to use flash, but allowing HTTP polling is required to get past old firewalls, etc.