Fix clip for closed polygons.

This pull request fixes a long standing bug (#443) by making polygon.clip work on closed polygons; previously it only worked on open polygons. The other polygon methods, area and centroid, already work on both open and closed polygons.

This pull request also changes d3.geom.polygon to use a prototype rather than creating every method as a closure. This will potentially break callers that assumed polygon methods were bound to the object instance, but this was never documented as an API guarantee so I am comfortable making this change in a patch release. (Also, the majority of examples using polygon.clip should now be using the more-convenient voronoi.clipExtent.)