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.
Got rid of the socket-level ping and added a test for the periodic callback with sinon. Let me know if you want me to change anything else. Thanks!