code: function parent(/*selector*/) {
    var arr = [], $ = this.air, slice = this.slice;
    this.each(function(el) {
      arr = arr.concat(slice.call($(el.parentNode).dom));
    });
    return $(arr);
  }
docs: Get the parent of each element in the current set of matched elements,
  optionally filtered by a selector.
  
  TODO: implement selector filtering