Support for namespaced model names

[pilaf]

Currently when using has_portal and referencing a model in the same namespace you need to also pass :class_name and :portal_key manually as the gem can't figure out the Ruby namespaces. E.g.:

module App
  class Family < FmRest::Spyke::Base
    has_portal :member, class_name: "App::Member", portal_key: "Family_Member"
  end
end

Let's also look at how ActiveRecord's relations deal with this... maybe we don't want to default :class_name to same namespace after all 🤔

We probably do want to strip the namespace for the default :portal_key though.

A similar thing happens with the default layout, which currently defaults to the class name, but includes the namespace. E.g.:

module App
  class Family < FmRest::Spyke::Base
  end
end

App::Family.layout # => "App::Family"

We probably want it to default to the class name with namespace removed, e.g. just "Family" in the above example.