v1.0.0

• Generate more valid polygons and multipolygons using pyclipper library for v2 MVT compliance (but we're still not fully v2 compliant for other reasons).
• Handle edge cases where polygon buffer makes a multi-polygon, ensuring inner rings are dropped when subtracting them from the polygon would make it invalid, and not adding multipolygons as array elements for multipolygon constructor.
• Calculate area more properly by using PolyTree result from Clipper.
• Factor out polygon validity code into its own file.