replaceVertex, replaceEdge for the js api

I would love to see a `replaceVertex` and `replaceEdge` for the JS graph API. Currently, I use:
```javascript
db.vertexcollection.replace(...);
```

However, having a `replaceVertex` and `replaceEdge` would make things easier, because when working with the graph api, we won't have to break out of the graph mindset and switch to the document api.