start to implement wdtk-rdf
Merge remote-tracking branch 'origin/master' into rdf-serializer
Merge remote-tracking branch 'origin/master' into rdf-serializer
implement RdfSerializer class
Merge remote-tracking branch 'origin/master' into rdf-serializer
Merge remote-tracking branch 'origin/master' into rdf-serializer
Added dependency to wdtk-rdf to allow testing
reconstruct json-serializer; start implementation of visitors
	- setup RdfConverter and Serializer for direct writing in RdfConverter
	- implement visitors for RdfConverter
Reorganised RDF serialization

* Move all actual triple writing to new class RdfWriter (no more OpenRDF
"Statements" in the main code)
* Move URI constants and methods to new class Vocabulary
* Rename methods in a uniform way (most methods now "write" something)
* Change method visibility and reorder methods according to style guide
* New Enum PropertyContext to specify how a property is used

More changes to come.
Collect vocabulary types and documentation to Vocabulary class
Use MD5 hashes in hex format when creating URIs for value resources
New class for interpreting Wikimedia Language codes
New class for finding and storing property datatypes
Fix handling of EntityValues
Remove projectName parameter

In some cases, the class will need to know which Wiki it is used with,
but for now let's just implement this for Wikidata only.
Merge remote-tracking branch 'origin/master' into rdf-serializer
implement PropertyTypes class
remove unused imports
implement reference representation, changeson PropertyTypes
Partial implementation of value serialisation code

* New buffer object to store values for later serialization
* Initial implementation for serializing quantities
* Many architectural changes ... more to come