Deleted one redundant check of URI and added one typematching for sesame

I found that there is not need to check that property is URI as it is already passed as URI. But in the same time I think it is nice to remind users that with Sesame they can only pass Resource to subject instead of just isInstanceOf[Resource] error
The other more correct way to do this would be to have
```
 def makeTriple(s: Sesame#Node, p: Sesame#URI, o: Sesame#Node): Sesame#Triple =  
    s match { 
         case res:Resource=>  new StatementImpl(res, p, o)
         case literal  => {
             val bn = BNode()
             new Graph(Triple(bn,owl.sameAs,literal),
                                Triple(bn,p,o)) 
         }
```

That follows the RDF semantics correctly, but it breaks the signature.

The problem is that if you restrict yourself in the whole api to Resource literals, how do you then get your reasoning engine to work without unduly creating blank nodes all over the place. Clearly you need to do this with Sesame, but should it be mart of the underlying model?