Update ontology URLs for 3.0
Our ontology URLs now use a namespace that we manage, at http://tracker.api.gnome.org/. This domain won't expire and can redirect anywhere we want, so that it can always link to the correct documentation for the ontologies.
We are going to continue using the Nepomuk name in the ontologies that were developed by the Nepomuk project, despite changing the URL. This way we acknowledge that we have downstream changes to the Nepomuk ontologies but still provide a link to the original shared vocabularies.
Fixes #162 (closed) and #104 (closed)