message: Make bot can receive subscribe and unsubscribe notification
diff: @@ -30,10 +30,16 @@ function JabberBot(configuration) {
  GroupManager(config, xmpp, bot, controller);
  + function request_roster() {
  + let roster_stanza = new Stanza('iq', { 'from': config.client, 'type': 'get'});
  + roster_stanza.c('query', { xmlns: 'jabber:iq:roster'});
  + xmpp.conn.send(roster_stanza);
  + }
  +
  xmpp.on('online', function (data) {
  console.log('Connected with JID: ' + data.jid.user);
  console.log('Yes, I\'m connected!');
  -
  + request_roster();
  });
  xmpp.on('close', function () {
  