Polygon gamma

``polygon-gamma`` seems left-over from the conversion from XML. Additionally, this removes some unused tags.

old|new
![20](https://cloud.githubusercontent.com/assets/1190866/3892911/62e73614-2236-11e4-9ed7-82721d2f0c4a.png)![20](https://cloud.githubusercontent.com/assets/1190866/3892913/66375556-2236-11e4-96a5-ff5c25f8ec1a.png)
![22](https://cloud.githubusercontent.com/assets/1190866/3892916/69c5dac6-2236-11e4-98ce-5fc80ceba3e5.png)![22](https://cloud.githubusercontent.com/assets/1190866/3892917/6b7a36aa-2236-11e4-8207-6eeab6f0b033.png)

I tried a filter of way_pixels > 0.01, but it didn't improve performance or change appearance

Fixes #73
Superceeds #867
Cross-reference #204