Move relation remoting to loopback

/to @ritch  or @bajtos 
What is the reason for keeping this dummy methods around? Since they are used only for the purpose of remoting, I would expect they can be moved to loopback too?

Also I believe they are not needed with new strong-remoting at all. Something along this way should work:

```
modelFrom.remoteMethod(
  '__findById__' + relationName, 
  {
    // remoting metadata
  },
  modelFrom[relationName].findById
});
```

@ritch can you confirm?