RDF::URI#label #comment for Vocabulary entries

Based on the feature request https://github.com/ruby-rdf/rdf/issues/156
(and my own needs), I implemented a first version of #label and #comment
on a Vocabulary entry.

Open for critique, I can try to improve it if needed :-)

* Vocabulary entries
* also RDF properties (temporary comment and label definitions)
* raise error when called on RDF::URI that are not vocabulary entries