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 the `action_name` method, and I've added a simple adaper for Padrino.