Foxx.Repository should provide more methods

Currently `Foxx.Repository` doesn't have an `update` method. A possible `update` implementation could look like this:

```js
Foxx.Repository.extend({
  update: function(model, obj) {
    this.collection.update(model.forDB(), obj);
  }
});
```

It would be nice if `Foxx.Repository` covered as much of the raw [documents API](https://www.arangodb.org/manuals/current/HandlingDocuments.html) as possible.