allow to change the _destroy attribute by setting destroy_action.

I needed a way to nullify a relation instead of destroying the child so I added a way to specify a custom destroy attribute like so :
```coffeescript
    relations: [
      type: 'many'
      key: 'thumbnails'
      relatedModel: -> Entities.Thumbnail
      destroy_action: '_remove'
    ] 
```

So now I can just add to my Rails child model :
```ruby
  def _remove
    !gallery_id.nil?
  end 

  def _remove=(bool)
    update_attribute :gallery_id, nil if bool
  end
```