Pass Nav props to ul so css classes can be applied

Per @pieterv https://github.com/react-bootstrap/react-bootstrap/issues/101#issuecomment-45953758
The `this.transferPropsTo` will need to me moved to [L49](https://github.com/react-bootstrap/react-bootstrap/pull/107/files#diff-13c45ffda4e4797e7d7dc0e0b809e4baR49) so that line would end up looking like:
```javascript
return this.transferPropsTo(this.renderUl());
```

In its current position for the non navbar use-case it would end up transferring all props to both the `nav` and the `ul`.