Support more forms of toplevel DRS disjunction in DRS->OWL

[Kaljurand]

If each argument of the disjunction maps to OWL SubClassOf-axiom then they can be merged into a single SubClassOf-axiom and thus represented in OWL. See more in
http://answers.semanticweb.com/questions/22864/expressing-axiom-disjunction-in-owl