Add client to server ping

While there are a couple of lines handling the underlying socket timeout, at least on Openfire servers, the default behavior of this adapter eventually has the hubot instance timeout and then reconnect as a client to the underlying xmpp server every few minutes (depending on the timeout configuration of the server). This fix adds in a ping so that the hubot instance will stay connected indefinitely.
@markstory: true enough--socket-level keepalive seems like overkill if protocol-level keepalive is merged.

All the socket-level keepalive does is write out a blank tcp packet every `options.keepaliveInterval` milliseconds to the connected server, so the call to ping should handle both keeping the socket open and keeping the client registered. Want me to go ahead and make the change?