Fix #1386 by removing dependency of Devise for logout_path and logout_method.

This way the main app can define a logout path on its routes and it will just work.

Also, logout_method is now an application helper and have a fallback to a
default value of :delete. It will just work (showing a logout link and all)
if you just add a route like the following:

  delete 'sign_out', :to => 'sessions#destroy', as: 'logout'