Making GomXsd* have instantiable properties: Part I

Serialization process requires you have instantiable types, no
interfaces ones, as properties. This make inviable to
implement Xsd* interfaces and get access to parse XSD documents.

In this part GomXsd* classes, now no implement XsdSchema to
provide instantiable properties.

Next step implement Xsd* interfaces, by prefixing them using "I"
in order to avoid API conflicts.