center prompting message in log in page

I think center this message could makes the UI more user friendly.

Before  
![1](https://cloud.githubusercontent.com/assets/1020965/4280148/ac2cb17e-3d29-11e4-9868-44e60261ff1b.png)

After  
![2](https://cloud.githubusercontent.com/assets/1020965/4280147/ac285dd6-3d29-11e4-96a9-7e613096d426.png)