Allow passing joi params directly

It would be nice if we could just pass joi objects directly in params definitions:

```js
ctrl.get('/:potato', function (req, res) {
  res.json({potato: 'potato'});
})
.pathParam('potato', joi.string().required());
```

This PR allows passing joi objects to `pathParam` and `queryParam` directly without having to wrap them in an object as `type` property (`description` and `required` can be derived off the joi object already).

@moonglum, @fceller do you see any problems with adding this syntax? The change is fully backwards-compatible.