 response for presence request.
The event parameter passed on the callback function has defined following properties:
- <strong>channel</strong> [string]: the channel name
- <strong>result</strong> [string]: JSON string containing the presence response
- <strong>error</strong> [string]: the error description, if occurs, otherwise empty string</p>
<p>Example:</p>
<pre><code>ORTC.addEventListener('onPresence', function(e) {
    if (e.error != ""){
        alert('(Channel: '+e.channel+') Presence error: ' + e.