写一个能够自适应的登录页面

<div class="container">
    <div class="row">
        <div class="col-lg-offset-4 col-lg-4 col-md-offset-4 col-md-4 col-sm-offset-4 col-sm-4 col-xs-offset-4 col-xs-4">
            <div class="row"><h1>登录</h1></div>
            <form class="form">
                <div class="row">
                    <div class="input-group">
                        <span class="input-group-addon">用户名</span>
                        <input type="text" name="userName" class="form-control" placeholder="userName">
                    </div>
                </div>
                <div class="row">
                    <div class="input-group">
                        <span class="input-group-addon">密码</span>
                        <input type="password" name="password" class="form-control" placeholder="password">
                    </div>
                </div>
                <