Add preferred namespace URI for handling ontology collisions

[dgarijo]

Until we get a better way for annotating this, we need to add the capability for annotating the preferred namespace in case there are ontology collisions. For example if my ontology has a class Person which extends foaf:Person, the schema Person should resolve to whatever the ns URI is declared.

This should be added in the documentation. There may be more than one preferred ns (as you can have an ontology network)

By default, the ontology that is extending the general ontologies should be used.