Padrino Adapter

Hey there, thanks for this gem! 

I'm proposing to remove Rails as a direct dependency. The only assumption pundit does about the underlying framework is that the action name can be queried using  `params[:action_name]`.

If we instead assume the presence of an `action_name` method present on the controller, we can easily make Pundit "framework agnostic". Rails already has this method, and I've added in Pundit a simple adaper for Padrino (taken directly from [padrino-responders](https://github.com/nu7hatch/padrino-responders/blob/master/lib/padrino-responders/helpers/controller_helpers.rb#L15)).