Add SAML support to Wt::Auth
We're working on SAML support for SSO (in addition to the existing OAuth support in Wt::Auth). This issue tracks that.
Note: we're basing this on OpenSAML, which is Apache-licensed, and hence sadly not compatible with GPL v2. This means that derived works that use Wt with SAML support may not be distributed, unless a commercial Wt license is used.