/// <summary>
/// 选中/取消表单中的所有复选框
/// </summary>
/// <param name="fm">复选框所在的表单实例</param>
/// <param name="checked">取消还是选中复选框</param>
/// <returns></returns>
function CheckAll(fm, checked)
{
    var elements = fm.getElementsByTagName("input");
    for(var i = 0;i<elements.length;i++)
    {
        if(elements[i].type == "checkbox")
        {
            elements[i].checked = checked;
        }
    }
}