Check for clockwise and duplicated points when creating buildings

[robhawkes]

Right now there aren't any checks in place to ensure building outlines are going the right direction (clockwise for outer, anti-clockwise for inner/holes) and that there aren't any duplicated points. Being thorough about these checks should prevent rendering quirks and also help fix #124 at the same time.

Not having these checks doesn't cause many problems when using Mapzen tiles, but many buildings fail to render when using other (more raw) data sources like Overpass and PostGIS SQL.