Support the Link response header consistently with Anansi

[nevali]

Specifically:

• Once the Content-Location has been evaluated and confirmed, create resource triples for each Link header, where the subject is the Content-Location (or Request-URI if absent), the object is the linked resource, and the predicate is the link relation.
• The link relation itself should be rebased as a URI against a base of http://www.w3.org/1999/xhtml/vocab# (so rel=license becomes a predicate URI of http://www.w3.org/1999/xhtml/vocab#license)