Improve contributor RDF

[earle]

At present:

    <dc:contributor>
      <foaf:Person rdf:ID="Name">
        <foaf:nick>Name</foaf:nick>
      </foaf:Person>
    </dc:contributor>

Better:

    <dc:contributor rdf:about="http://openguides.example.org/wiki.cgi?id=Name;format=rdf" />

As the current design of the system (no specific user profile pages) stands, the output will be faintly semantically odd as it will imply that each person is a dc:contributor to themselves. But, I suppose there's some philosophical truth to that, and it's still better than the current output.

#248 will allow further improvement to:

    <dc:contributor rdf:about="http://openguides.example.org/page/Name" />

And if user profiles are ever implemented,

    <dc:contributor rdf:about="http://openguides.example.org/user/Name" />

which, with Content-Type negotiation, is the optimal case.

[earle]

(In [1166]) Link dc:contributor items in RDF output to actual URIs that exist in our system; closes #250.