Collection::add() and Collection::remove() methods

This pull request adds methods to add/remove records from a `Collection` in `relation.coffee` without affecting other records in the `Collection`.

Turns this:
```coffee
photo = album.photos().create()
photo.updateAttribute 'album_id', null
photo.updateAttribute 'album_id', album.id
```

Into this:
```coffee
photo = album.photos().create()
album.photos().remove photo
album.photos().add photo
```