Login cleanup

Way cleaner and modern looking log in and installation page. Check it out @owncloud/designers 

In the wake of that I also:
* cleaned up more CSS
* made the footer stick to the bottom
* enlarged the buttons
* enlarged the clickable area for the »remember« checkbox and »advanced« dropdown
* adjusted the primary button
* beautified the warnings
* made the inputs work on any background (also for theming)

Should work fine also in IE8 and up.