Add #model_name instance method to ActiveModel::Naming

This pull request addresses #15428 and [discussion on the Rails Core mailing list](https://groups.google.com/forum/#!topic/rubyonrails-core/ThSaXw9y1F8).
While I'm aware that this is one of the simplest ways to add the `#model_name` method to the extending class, I'm somewhat sad that it has to live directly on the class rather than sitting on the `Naming` module.

My personal preference would be to see a module mixed in, but it's obviously not a big deal.