Use an overlay over default_dirt.png for soil

#363.
~~I don't really like the textures, though (e.g. the red-ish wet soil top). Could someone supply different textures?~~
![new_soil2](https://cloud.githubusercontent.com/assets/2159710/5332123/0ec0d7aa-7e4a-11e4-8c02-e56121653c76.png)