Refactor of ::Rails module

1. Used `ActiveSupport::Autoload` to dry-up the `autoload` definitions.
2. Used `ActiveSupport`'s `delegate` to dry up the application proxied attributes. This actually let me to the introduction of the `config` attribute and making `configuration` an alias to it. I think that the `config` attribute is a nice addition, but you may disagree.
3. Did a little white space change on `Rails.groups`.