change tests for offline execution of fetchPropertyType()
introduce interface for PropertyTypes classes
	- new interface PropertyTypes
	- rename class PropertyTypes to PropertyTypesImpl (for WikiData
		Properties)
	- put Url of the WebApi of Wikidata as a constant in PropertyTypesImpl
add licence header and javadoc to PropetyTypes
implement tests for RdfConverter
Merge branch 'rdf-serializer' of https://github.com/Wikidata/Wikidata-Toolkit into create-parser-of-constraints