Only output one <rdf:Description> for each node

The following RDF/XML part:
```xml
  <rdf:Description rdf:about="http://zoggy.github.io/ocaml-rdf/">
    <dc:title>OCaml-RDF</dc:title>
  </rdf:Description>
  <rdf:Description rdf:about="http://zoggy.github.io/ocaml-rdf/">
    <dc:description>
      OCaml-RDF is an OCaml library to manipulate RDF
      graphs and execute Sparql queries.
    </dc:description>
  </rdf:Description>
```
describes exactly the same graph as:
```xml
  <rdf:Description rdf:about="http://zoggy.github.io/ocaml-rdf/">
    <dc:title>OCaml-RDF</dc:title>
    <dc:description>
      OCaml-RDF is an OCaml library to manipulate RDF
      graphs and execute Sparql queries.
    </dc:description>
  </rdf:Description>
```
The latter is less verbose, more idiomatic and more human-readable. This patch changes ```Rdf_xml.output``` so that only one ```<rdf:Description>``` is created for each subject node in the graph.