EasyUI 创建Tree

tree可以被从标记创建。easyui tree应该定义在ul元素中。无序列表ul元素提供了基本tree结构。每一个li元素被产生一个tree节点，子ul元素产生父tree节点。
例子：

    <ul id="tt">

        <li>

            <span>Folder</span>

            <ul>

                <li>

                    <span>Sub Folder 1</span>

                    <ul>

                        <li><span>File 11</span></li>

                        <li><span>File 12</span></li>

                        <li><span>File 13</span></li>

                    </ul>

                </li>

                <li><span>File 2</span></li>

                <li><span>File 3</span></li>

            </ul>

        </li>

        <li><span>File21</span></li>

    </ul>

创建tree:
$('#tt').tree();
显示：