Add option to make `remember_token` cookie secure

This is important if you have an app running on HTTPS, otherwise the
auth cookie is leaked when you visit a HTTP URL and can be intercepted.

Read more:

* http://blog.teamtreehouse.com/how-to-create-totally-secure-cookies
* http://guides.rubyonrails.org/action_controller_overview.html

Also:

* Fix documentation of `Clearance::Backdoor` (was missing namespace).