#35 Provide <audio> or <video> elements to MediaHandler - first cut

 Usage:

  var session = new SIP.UA().invite('welcome@junctionnetworks.com', {
    media: {
      render: {
        remote: {
          audio: document.createElement('audio'),
          video: document.createElement('video')
        },
        local: {
          audio: document.createElement('audio'),
          video: document.createElement('video')
        }
      }
    }
  });