Fix #issue101
Merge pull request #102 from Wikidata/issue-101

This fixes #101. In particular, it removes duplicate triples from RDF exports, thereby making RDF exporing about twice as fast (less than 3h for all exports).
Merge branch 'master' of https://github.com/Wikidata/Wikidata-Toolkit into create-classes-to-measure-the-memory-usage