Remove validity constraint from polygons

As [discussed](https://github.com/GeoJSONWG/draft-geojson/pull/3#issuecomment-17956266) in #3, we don't want to enforce that GeoJSON geometries be valid.

This branch also cherry-picks the typo fixes that came with 424a06e43b7c5f3beb03d52684d97540f8389c65 (which was reverted in the batch with b4d6c641a84eb50d49cd50f1983bf06dd6659181).