added support for optional_attributes

[GavinJoyce]

This adds support for optional attributes:

class PersonSerializer
    include RestPack::Serializer

    attributes :id, :name
    optional :description
end

By default, description won't be serialized. To force inclusion:

PersonSerializer.as_json(album, { include_description?: true })

closes #116

TODO:

• Update the readme

/fyi @eugeneius

[eugeneius]

awesome 🎉