improve login page styling

Not that I don't like Min's taste, but I feel we can do better on the login page.
I'm no designer, but this feel a bit prettier:
![capture d ecran 2014-09-21 a 18 27 38](https://cloud.githubusercontent.com/assets/335567/4349215/6694214c-41ac-11e4-9c24-2305f55d77ab.png)

For instance, the global `width: 100%` on  `.input-group` css below shouldn't be done globally for all input groups on all pages. It only makes sense if it's scope limited to this particular form. There are several such things here.