removed useless checks in equals()
Refactored property hierarchy to remove generics. Introduced Searcher.

A large amount of generics in OWLPropertyExpression was there only
to support convenience methods to search through ontologies.
Simplified this by removing the convenience methods and introduced a
class to write customizable searches.

Searcher is still embryonal. It currently relies on convenience 
methods in OWLOntology. It should evolve to use ontology internals
to simplify and speed up searches and filters.