- In GeoIP handler constructors that take a string (to configure the
selector to use), interpret the empty string (which generally isn't
a meaningful selector) as meaning "use the default".
- Drop the no-argument constructors in favor of a default-argument
which is empty.
- GeoIP gets a string selector; the interpretation is up to derived classes.
- GeoIPXML and GeoIPJSON use the selector to select an element by tag
or an attribute, respectively.
- The handler for JSON data should be called that, not named
specially after the original provider it was implemented for.
- Make filename and classname consistent, GeoIPJSON.
