add support for writing spatial filters in an SLD document

patch implementing spatial filters and writing them in SLD

The Trac view of the patch does not show all files affected (download in original format does).

Would be great if someone could review this sometime.

One thing I did not like is that I had to use an OpenLayers.Geometry.Rectangle (which is about to be deprecated) to represent a gml:Box. The comments state one should use a OpenLayers.Bounds.toGeometry() which will yield a Polygon, but in that case I think we would need a utility function on a Polygon to see if it is a rectangle or not, to represent it in its right GML form.

Also, is it necessary to create OpenLayers.Format.GML objects like I do now to serialize the geometries, or can we inherit Format.SLD from Format.GML?