function loadData(locations, response, callback) {
      if (locations.length === 0) callback(null, response);

      else $.get(locations.shift())
        .fail(function(e) {
          callback(e, null);
        })
        .done(function (data) {
          if (response.length > 0) response += '\n\n';
          response += data;
          loadData(locations, response, callback);
        });
    }
File fetcher function.

Fetches a given `url` via AJAX.
See [Runner#run()] for a description of fetcher functions.