Don't cache any literals in OWLDataFactoryInternalsImpl.

Every  call to any of the  getOWLLiteral method was building a brand new instance  anyway, even if there would have been a cache hit.

Overall improvement of about 9% when parsing Gene Ontology in FSS.  (mostly xsd:string literals)
Merge pull request #194 from sesuncedu/dont_cache_literals

Don't cache literals cos it's literally useless