WebRTC.MediaStreamManager.render: allow media elements to be functions

This way, you can use a renderHint without needing media elements ahead of time:

var ua = new SIP.UA({traceSip: true});
var session = ua.invite('welcome@onsip.com', {
  media: {
    render: {
      remote: {
        audio: function () {
          return document.createElement('audio');
        }
      }
    }
  }
});

Addresses #99 as described in https://github.com/onsip/SIP.js/issues/99#issuecomment-56213485