Upgrade `omniauth-saml` to 1.5.0 and document it's new capabilities.

With this MR I'm upgrading `omniauth-saml` to 1.5.0 to include new and improved functionality. See https://github.com/omniauth/omniauth-saml/blob/master/CHANGELOG.md for more details.

This MR includes new documentation, so @axil can you also take a look?

/cc @DouweM 

See merge request !3170