下拉菜单必要的代码：
<div class="container">
<div class="dropdown">
<button class="btn btn-default dropdown-toggle" id="aa" data-toggle="dropdown" aria-expanded="true">
按钮</button>
<ul class="dropdown-menu" role="menu" aria-labelledby="aa">
<li><a href="">菜单1</a></li>
<li><a href="">菜单2</a></li>
</ul>
</div>
</div>
可在下拉列表中插入的内容有：a,分割线：<span class="glyphicon glyphicon-xxx" aria-hidden="true"></span>
b,提示信息：<li role="presentation" class="divider"></li>
c,默认不能点击：<li role="presentation" class="disabled"><a href="jarascript:voidco">xx</a></li>