extracts the property value out of an itemprop DOM node
function parsePropertyValue (node) {
    var $node = $(node);

    if ($node.is('meta')) {
      return resolveAttribute($node, 'content');
    } else if ($node.is('audio,embed,iframe,img,source,track,video')) {
      return resolveUrlAttribute($node, 'src');
    } else if ($node.is('a,area,link')) {
      return resolveUrlAttribute($node, 'href');
    } else if ($node.is('object')) {
      return resolveUrlAttribute($node, 'data');
    } else if ($node.is('data,meter')) {
      return resolveAttribute($node, 'value');
    } else if ($node.is('time')) {
      return resolveAttribute($node, 'datetime');
    } else {
      var text = $node.text();
      return text || '';
    }
  }