Add support for Serializer.can_include(:model)

As per https://github.com/RestPack/restpack-serializer/issues/4, this allows a whitelist of valid includes to be defined in a Serializer:

```ruby
class AlbumSerializer
  include RestPack::Serializer
  attributes :id, :title, :year, :artist_id

  can_include :artist, :songs 
end
```