bootstrap button按钮长度

与父容器等宽
　　To use the full width of the container within which the button is residing, Bootstrap 3 offers block button option.
　　使用按钮的block样式，让其与父容器等宽
　　1160x178
<div class="container"> 
<div class="row">
<div class="col-lg-12"> 
<p> 
<button type="button" class="btn btn-primary btn-sm btn-block">Default small block level button</button> 
<button type="button" class="btn btn-default btn-lg btn-block">Default lg block level button</button> 
<button type="button" class="btn btn-primary btn-lg btn-block">Default lg block level button</button> 
<button type="button" class="btn btn-primary btn-xs btn-block">Default extra block level button</button> 
</p> 
</div>
</div>
</div>