Supporting http://tools.ietf.org/html/draft-ietf-xmpp-websocket-00

Added support for WebSocket subprotocol 'xmpp' which is expected by some libraries such as Strophe, switched to XMPP over WebSocket standards (http://tools.ietf.org/html/draft-ietf-xmpp-websocket-00) from RFC 7395 since most libraries implement <stream:stream> instead of xmpp-framing <open xmlns='urn:ietf:params:xml:ns:xmpp-framing' />