Use vanilla before_filter in example subclass

[foxyblocks]

In the readme example under Customizing controller behavior we use a custom authentication method authenticate_user! only: [:morph]. This method that is not really explained. We should change it to a vanilla example. Something like:

class MorphingController < Morphing::MorphingController
   before_filter :authorize_admin

   private

   def authorize_admin
     unless user.is_admin?
       flash[:error] = 'Permission denied!'
       redirect_to root_path
     end
   end
end