Moved signature helper to an extension in AuthServices and not only in the tets.
- Did some work on the Saml2Response_Xml_FromData_IsSigned test - still not enabled though as it doesn't pass.
- Removed Saml2Namespaces.DsigName in favour of SignedXml.XmlDsigNamespaceUrl.
- Added the XmlDocumentExtensions.Sign() method, including tests.