Poly2tri

Please review :) 

Compared to jts conforming delaunay, poly2tri only adds vertices where necessary for triangulation so it results in much less vertices which reduces obj file size and  less vertices to interpolate which should make it faster. Also the results look better on complex roof as the interpolation finds not the best segments sometimes (or there are none) 

[roof_poly2tri](https://f.cloud.github.com/assets/1747/405520/7e878fc2-a9e7-11e2-8037-c99cc35028d9.png)
vertices: 2266, faces: 3773

![roofs_conforming](https://f.cloud.github.com/assets/1747/405521/921d2da8-a9e7-11e2-9069-b50634c55a9d.png)
vertices: 3124, faces: 5203

http://city.informatik.uni-bremen.de/~jeff/dom8.osm