rewrote RDF serialization to use a much simplified Turtle format

instead of ranked label+concept results, all we really need is an
alphabetical list of labels

the intent here is to simplify adoption by not requiring API users to
learn about RDF first (and thus not requiring an RDF parser)

NB:
* Since IqRdf appears unable to generate the desired list of objects,
  we're manually generating the simplified Turtle format - which also
  means that XML/RDF is no longer available.
* The HTML representation still uses ranked label+concept results.