Use a base URI for all reference to local SPDX elements

[goneall]

Current implementation of the parser uses rdf:about with a full URL - this makes the SPDX RDF document rather bulky.

Use a base URI will allow rdf:id= with short identifiers.

Code snippet available at http://stackoverflow.com/questions/715287/add-xmlbase-to-xml-file-in-java

Copied from Bugzilla Bug 1219