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.