Allow to add connection with through data for HasManyThrough relation

e.g.:

```js
physician.patients.add(patient, { date: new Date(now) }, function (e, app) {
   ...
});
```

Signed-off-by: Clark Wang <clark.wangs@gmail.com>
@raymondfeng the remote definition I altered in **loopback** also invoke **referencesMany.prototype.add()**. I can't find any document about this kind of relation, so I'm not sure what to do with the `data`.