Improved labels and inputs layout in admin login

As explained and proved in #19177 (https://code.djangoproject.com/ticket/19177), layout looks odd/bad in several languages.
Also, it looks like and old design, a renew is needed here. So, I propose this change following claudep sugestion.

- Changes were made just in the CSS.
- Modified layout to top-aligned labels.
- Added more padding to inputs (it was really needed here).
- It adjust good to zoom in/out.

To make inputs fit perfect, it uses the box-sizing property, with the current support:
Chrome (any): box-sizing
Opera 8.5+: box-sizing
Firefox (any): -moz-box-sizing
Safari 3: -webkit-box-sizing (unprefixed in 5.1+ versions)
IE8+: box-sizing

![Captura de pantalla 2013-02-23 a las 15 08 27](https://f.cloud.github.com/assets/844554/188362/147b9f9e-7de4-11e2-9c3d-f5036f89e1d4.png)