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.