add support for SchemaData schemaurl

[brownaa]

I have a KML file which has been exported from another system which translates all of the meta data about each point into the <ExtendedData> tag. This information can be accessed but the schemaurl remains inaccessible through this package.

In this case, it is necessary to know what the whole entire extended schema is to be sure to structure data retrieval/extraction properly.

<ExtendedData> <SchemaData schemaUrl="#kml_schema_ft_obj"> <SimpleData name="id">1234</SimpleData> <SimpleData name="total_well_depth">2468</SimpleData> <SimpleData name="owner">SEIFERT</SimpleData> </SchemaData> </ExtendedData>

In the case above, I need to access all of the elements of the schema where the id is kml_schema_ft_obj.