Improve data generalization

[dmitrig01]

• make sure the polygons for a zoom level aren't overly detailed. this is currently happening with zoom level 6 for example
• fix clipping; this introduces very weird artifacts that are visible in our mapnik-rendered image tiles, but especially apparent when zooming in
• find other ways to reduce the buffer size; oscim's data sizes are about 20x smaller than ours!? (when zooming into a particular point from the world view)
• don't reencode the street lines again for label layers; find a more efficient way to define label layers

[yhahn]

@kkaefer @dmitrig01 you may want to look at the v4 vector tiles which have some improvements to polygon selection (no changes to generalization/simplification though).

http://a.tiles.mapbox.com/v3/mapbox.mapbox-streets-v4.json

This ticket for more info: https://github.com/mapbox/mapbox-streets/issues/605

[kkaefer]

closing in favor of #48