title: subscription error in subscribing to a pusher presence channel
body: <p>I'm trying to go through a basic presence channel example with pusher.js and I'm getting a <code>pusher subscription_error , invalid auth response for channel, expected channel_data field</code></p>
  
  <pre><code>    var presenceChannel = pusher.subscribe('presence-' + room);
  
      presenceChannel.bind_all(function(err) {
          console.log("err: " + err);
  
      });
  
      presenceChannel.bind('pusher:subscription_succeeded', function(members) {
          members.each(function(member) {
              console.log(member);
          });
      });
  </code></pre>
  
  <p>Is there some sort of initialization that I need to do to create a presence channel beforehand?  Or can I just connect to one and it will create a presence channel.  Does subscribing to a presence channel add my presence info to it as a member?</p>