Multithreading, better polygons, smaller memory footprint, and other improvements

• Tile generation is multithreaded to take advantage of multiple CPUs
• More compact data representation reduces memory usage and improves speed
• Polygon clipping uses Clipper
  and makes sure interior and exterior rings are distinguished by winding order
• Individual GeoJSON features can specify their own minzoom and maxzoom
• New tile-join utility can add new properties from a CSV file to an existing tileset
• Feature coalescing, line-reversing, and reordering by attribute are now options, not defaults
• Output of decode utility is now in GeoJSON format
• Tile generation with a minzoom spends less time on unused lower zoom levels
• Bare geometries without a Feature wrapper are accepted
• Default tile resolution is 4096 units at all zooms since renderers assume it