Added a LinkedData origin. Uses HEAD request to probe content type

Added an initial Linked Data origin, just does a HEAD request to test if content type delivered from the URL is one of the supported RDF serializations. This should fix #36.

There's still a bit of an inconsistency between this and the publishing format though, as the latter may also attempt to use autodiscovery. In that case the origin will be :html rather than :linkeddata, but the format will still be :linkeddata, which I think makes some sense.