render amenity=doctors

Symbol is from SJJB icon set (http://www.sjjb.co.uk/mapicons/), license CC-0.
Solves issue #89 .
Tried to make a usable .svg symbol instead of the .png, but failed miserably.
![amenity-doctors-rendered](https://cloud.githubusercontent.com/assets/4013374/4603042/f93b9636-5158-11e4-96d8-05f425709851.png)