parse XRD and host-meta files into RDF::Trine models

 Resource Description Framework (RDF) is a standard model for data
 interchange on the Web.
 .
 Extensible Resource Descriptor (XRD) is an XML format for discovery of
 metadata about a resource – in particular discovery of services
 associated with the resource. It is commonly used in relation to
 OpenID, YADIS and OAuth.
 .
 While XRD has a rather different history, it turns out it can mostly be
 thought of as a serialisation format for a limited subset of RDF.
 .
 This package ignores the order of <Link> elements, as RDF is a graph
 format with no concept of statements coming in an "order". The XRD spec
 says that grokking the order of <Link> elements is only a SHOULD. That
 said, if you're concerned about the order of <Link> elements, the
 callback routines allowed by this package may be of use.
 .
 This package aims to be roughly compatible with RDF::RDFa::Parser's
 interface.