Cleaned up and restructured code

Smaller classes with more clearly defined tasks.
Merge branch 'rdf-serializer' of https://github.com/Wikidata/Wikidata-Toolkit.git into rdf-serializer
implement test for writeInstanceOfStatements
Rewritten Linked Open Data exports

* Export links in addition rather than instead of Wikidata contents
* Manage link exports through a flexible system extension classes that
register on certain properties
Merge branch 'rdf-serializer' of https://github.com/Wikidata/Wikidata-Toolkit.git into rdf-serializer
Mention new RDF export in release notes
Merge pull request #75 from Wikidata/rdf-serializer

Rdf serializer
Merge branch 'master' of https://github.com/Wikidata/Wikidata-Toolkit into create-parser-of-constraints