Format.GML: introduce option to postpone parsing of the geometries

Patch implementing extractGeometry APIProperty on Format.GML

I've attached a proposed patch. Things which need discussion:

1) is it okay to add properties to OpenLayers.Feature.Vector which are not advertized in that class? I want to cache the geometryNode, and I named the property feature.geometryNode. But I don't know if this is allowed without specifying it in Vector.js.

2) maybe it's necessary to create another APIProperty called cacheGeometryNode? Now by default I cache the node when parseGeometry is false, but maybe this is not flexible enough and we need an extra property to control the caching?

An example usage is:

            var gmlParser = new OpenLayers.Format.GML();
            var feature = row.get('feature');
            if (!feature.geometry) {
                var type = feature.geometryNode.type;
                var parser = gmlParser.parseGeometry[type.toLowerCase()];
                if(parser) {
                    var geometry = parser.apply(gmlParser,
                        [feature.geometryNode.node]);
                    feature.geometry = geometry;
                    this.layer.addFeatures(feature);
                }
            } else {
                this.layer.addFeatures(feature);
            }

The answer to the former is 'Yes, that's fine', the answer to the latter is, in my mind, 'no, we don't need that.'