Support IdP SLO (IdP initiated logout)

[drnic]

https://github.com/calh/ruby-saml-rails3-example supports IdP initiated logout (via an unmerged patch to ruby-saml, ugh, https://github.com/onelogin/ruby-saml/pull/26).

It'd be nice for this canonical IdP written in ruby to speak the SLO sequence.

I created an example rails app https://github.com/drnic/ruby-saml-idp-rails3-example to wrap around ruby-saml-idp. I've added @lawrencepit as admin to it :)