Helper methods for groups of mappings

This provides the ability to define helper methods relative to groups of mappings.
For example if both User and Moderator can comment things you might need something like current_commenter, authenticate_commenter! or commenter_signed_in? to use in your CommentsController or in the views. This will do the trick for you:
```
# config/routes.rb
devise_group :commenters, includes: [:users, :moderators]
```
(I didn't implement the session method because.. I didn't really know what to do, or whether it could be useful or not)