New linear-gradient syntax

http://www.broken-links.com/2012/01/11/the-new-and-hopefully-final-linear-gradient-syntax/

As @gitbase mentioned in #77, it should be:

``` css
background-image: linear-gradient(to bottom, #444444, #999999);
```

…instead of:

``` css
background-image: linear-gradient(top, #444444, #999999);
```
