Don't include polymorphic belongs_to in links
Add support for polymorphic links

Uses JSON API's linked resource object format to represent polymorphic
belongs_to relations.
Merge branch 'add-polymorphic-links' into dev

Conflicts:
	lib/restpack_serializer/serializable/side_loading.rb
	spec/fixtures/db.rb
	spec/fixtures/serializers.rb
	spec/serializable/side_loading/side_loading_spec.rb