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.
If you have the time, that would be great, if not I can take care of it post-merge.