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)

otherwise you get a 2px border in between username/password field, so -1px margin. I can also try the `border-bottom:none` but then I have to set it on 4 or 5 elements IIRC.