When runnin w/in Rails, make available url_helpers

This technique ties this usage to Rails, which but makes the helpers
directly accessible w/in any instance of ActiveModel::Serializer,
including any associations.
Inject explict UrlGenerator into serializers

This is done automagically for Rails, but non-Rails apps can pass along
their own url generators/helpers to be used within serialzers.
Allow configuring default_url_options AMS-wide

This is useful when running either outside of Rails, or in a background
job where you won't have access to the current url options for a
request.
Run load hooks to finish configuration