fixed typo
added comments
fixed test data: only URIs are allowed as predicates (#13)
added tests (#13)
improved documentation (#13)
implemented data type and language extraction (#13)
changed assertion to improve information in case of a failure (#13)
improved failure message (#13)
improved message formatting
added test stubs (#13)
added comments (#13)
implemented tests (#13)
added test (#13)
fixed test (#13)
added workaround to overcome problems with strings and untyped literals (#13)