Added size-based culling and added documentation
Added size-based culling and added documentation

SEL-Columbia/TileStache,mojodna/TileStache,Blechhirn/TileStache,Kosta-Github/TileStache,Billups/TileStache,mpuig/TileStache,mpuig/TileStache,TileStache/TileStache,Blechhirn/TileStache,Billups/TileStache,mapzen/TileStache,moskvax/TileStache,SEL-Columbia/TileStache,MetGIS/TileStache,mapzen/TileStache,robpvn/Vector-Tile-Research,Kosta-Github/TileStache,Billups/TileStache,TileStache/TileStache,robpvn/Vector-Tile-Research,jbants/TileStache,MetGIS/TileStache,TileStache/TileStache,mapzen/TileStache,robpvn/Vector-Tile-Research,Blechhirn/TileStache,mojodna/TileStache,moskvax/TileStache,moskvax/TileStache,jbants/TileStache,Kosta-Github/TileStache,jbants/TileStache,SEL-Columbia/TileStache,MetGIS/TileStache