RDF serializer fix
- implement support for Rdf conversion of statements with properties
which have MonolingualText as primitive type
add licence header
implement translation from wikidata language encoding to BCP 47
add test for MonolingualTextValueConverter
remove space character from JSON datatype IRI
Use one method to convert MTV values to RDF (not two different ones)
Work with JSON dump by default
Guard against properties having been deleted since dump (no datatype)
Merge pull request #99 from Wikidata/monolingualTextValueSupportBugFix

Monolingual text value support in RDF export
Always clear class declaration buffer after adding declarations
Insert a safety check for null strings

This should be avoided by the data model implementation, but there seem
to be some issues with this under yet-to-be-identified conditions in the
JSON dumps.
Include method for registering new site information in Sites

This is useful to add missing sites information after loading the sites
table (which can happen if the sites table dump is older than the data
dump).
Document assumptions on site link URLs
Add some default values to JSON implementations
Use modern entity timer and file paths

Improved compatibility with code in ExampleHelpers.
Document bug fixes
Merge pull request #100 from Wikidata/rdf-export-fixes

Rdf export fixes
Merge branch 'master' of https://github.com/Wikidata/Wikidata-Toolkit into serialization-of-constraints