code: function getFeed(url, callback) {
    getXmlString(url).then(xmlStringToJSON).then(callback).catch(callback);
  }
docs: Function to get an Atom/RSS feed.
  @param {string} url - url to rss feed, this url must resolve to something that returns via http the xml itself.
  @param {function} callback - the function to call back upon with the Feed object.