refactoring: extracted base class for test helpers (#28)
extracted base class for SPARQL connector tests (#28)
added class stub for Stardog connector (#28)
improved Stardog test helper (#28)
added test case for Stardog SPARQL connector (#28)
configured SPARQL connector (#28)
test helper clears database whenever the SPARQL connector is requested (#28)
implemented Stardog connector partially (#28)
added special handling for queries that return a boolean value (ASK, DELETE, ...) (#28)