Add support for "EMPTY" WKT and WKB

[larsbutler]

WKB/WKT parsers/writers need to support empty geometries.

NOTE: WKB does not support POINT EMPTY. When converting an empty point to WKB, an exception should be raised.

Open issue: What's the best way to represent empty geometries in GeoJSON? There doesn't seem to be anything directly supported in the spec: http://geojson.org/geojson-spec.html

[larsbutler]

@sgillies Question for you: What's the preferred way of representing empty geometries in GeoJSON? It's not covered in the specification. For the branch I'm working on right now, I'm just treating an empty coordinates list as "EMPTY". Is there a better or more ubiquitous way?