Add read support for PostGIS.

Now Oracle's MDSYS.SDO_GEOMETRY type can be translated to a PostGIS "geometry" automatically.
This is done by using WKB (well-known binary) format for data transfer, which is not ideal for performance as Oracle's SDO_UTIL.TO_WKBGEOMETRY isn't, but avoids messing with internals.

Thanks to Vincent Picavet and Vincent Mora for their help!

Still missing:
- RETURNING support for geometries.
- INSERT and UPDATE of geometries.