修复大屏幕主栏过窄

修改前：

![image](https://cloud.githubusercontent.com/assets/1382065/2688458/5c0b80ec-c2a1-11e3-863c-ecee72d692d2.png)

修改后：

![image](https://cloud.githubusercontent.com/assets/1382065/2688460/7f8212ac-c2a1-11e3-8bc9-66c89bc01d7e.png)