Standard DBRef support

As mentioned in #489 and #491, Refs in mongoose are not standard DBRefs.

This pull request is not finished, unit tests have to be added and checked and one TODO is still on the list, but the solution has worked for me so far. Populating works as expected and the values written back to the DB now are standard DBRefs (i.e.:

```javascript
    {
      "$ref": "fooCollection",
      "$id": ObjectId("4e5f7a73326a0206c9400000"),
      "$db": "barDB"
    }
```