Make url_helpers available in serializers

This is built for 0.9, and is a first-pass at getting Rails' `url_helpers` into the serializers. The current approach is a bit tied to Rails, but makes the helpers directly accessible w/in any instance of `ActiveModel::Serializer`. This also works for associations.

An idea I have for an alternative approach is to expose the URL helpers via a `urls` method available from serializer instances, rather than mixing the `url_helpers` directly into `ActiveModel::Serializer`. This would at least provide a seam for non-Rails apps to implement their own solution, while still relying on a `Serializer.urls` API. I could try that here to in a different pull request, if it's desired. Or we could roll with this for now and try that when/if needed.

Thoughts?