Convert to using draft-xmpp-websocket-01

The latest version of the XMPP over WebSocket spec changes the `<stream:stream>` and `</stream:stream>` messages to be `<open />` and `<close />`, respectively.

This is a breaking protocol change that requires a matching server implementation. An updated Prosody `mod_websocket` will be published shortly now that we've tested that server+client works.