Load the next page of items
@return {void}
function() {
  var tree = this.tree;
  var scrollTop = tree.scrollTop;
  var scrollHeight = tree.scrollHeight;
  var offsetHeight = tree.offsetHeight;
  var atBottom = scrollTop >= (scrollHeight - offsetHeight);

  if ((atBottom && this.pageIndex < this.pages.length)) {
    var f = document.createDocumentFragment();

    util.each(this.pages[this.pageIndex], function(i, item) {
      appendItem(item, f, this.customOption);
    }, this);

    tree.appendChild(f);

    this.pageIndex++;

    this.emit("selectr.paginate", {
      items: this.items.length,
      total: this.data.length,
      page: this.pageIndex,
      pages: this.pages.length
    });
  }
}