Minor fixes at XML serialization tests for INSPIRE objects.
Renamed Object.lookup_schema classmethod
Merged with smanousopoulos/master (INSPIRE XSDs, tests)
Renamed tests for INSPIRE XML serializers
Reviewed INSPIRE XML serializers (to_xsd()). Must fix XSD paths.
Rewrote key-tuple serializer (accepts a prefix)
Minor fix at key-tuple serializer (accept non-empty prefix)
Fixed #37
Added several opts to schema flattening methods
fixed minor tests issues