fix for wrong diesel resolution

This PR resolves #207  Added two implicits to diesel, so Rdf#URI and Rdf#BNode should now be resolved out of the box.
Sooo, this is more complex than it seems...

>  I tried `val alex: Rdf#Node = URI("http://bertails.org/#betehess")`

Yes, if your type is behind a parameterized `Rdf`, then it's fine.

My remark was with something like that:

```
val g: PointedGraph[Plantain] = (
  bnode("betehess") -- rdf.first ->- "Alexandre"
)
```

Your solution makes it work but somehow, this does not compile anymore:

```
val gg: PointedGraph[Plantain] = (
  (bnode("betehess"): Plantain#Node) -- rdf.first ->- "Alexandre"
)
```

This is equivalent to 

```
  val ggg: PointedGraph[Plantain] = (
    model.BNode("betehess") -- rdf.first ->- "Alexandre"
  )
```

which is the issue we basically had at the time when using Jena methods directly. Here, `bnode` somehow keeps the type without aliasing it. I have no idea why this is happening.

To be honest, your solution still puzzles me a lot, because `Plantain#BNode` is a subtype of `Plantain#Node`, so the two implicits should be applicable, resulting in none of them being chosen because of the ambiguity.

Now, as expected, doing the following makes everything works:

```
implicit def foo(node: Plantain#Node) = diesel.toPointedGraphW[Plantain](node)
implicit def bar(pointed: PointedGraph[Plantain]) = diesel.toPointedGraphW[Plantain](pointed)
```

This is a strong argument in favor of having the implicits directly available in `RDFOps`. 

> what does "W" mean in the end of classes and methods?

It is just a convention for **W**rapper.