Add very simple style for file 'buildings.shp'

At least for Denmark Geofabrik includes buildings in a file called 'buildings.shp'. (See http://download.geofabrik.de/europe/denmark-latest.shp.zip). I just added an extremely simple style for these. 
![image](https://f.cloud.github.com/assets/1392735/366252/f29bf95c-a27b-11e2-86c0-6a0b2802f33f.png)