Update RDF::Util::File.open_file to use Net::HTTP, follow redirects, and set :base_uri appropriately.
This completes work on issue #136
Merged 'develop'.
When fetching files with Accept header, reduce priority of text/plain and text/html to favor formats that are unequivacally RDF.
Add more vocabulary definitions:gr, ical, og, prov, skosxl, v, vcard, void, wdrs, and xhv.
Other vocabularies are made StrictVocabulary and have updated contents based on OWL/RDFS definition of vocabulary.