Rebased from original master
Updated test XmlDocumentExtensions_Sign
- Now checks that a valid reference is used in the signing.
- Removed dangling param in xml comment causing compilation errors.
Minor fixes to Saml2ResponseTests.

- Readded an xml declaration that had been removed.
- Changed Status:Requester to Status:Success for all test messages containing assertions.
Cleanups to Saml2Response
- Removed this prefix for member access.
- Made allAssertionElementNodes lazily evaluated.
Merge pull request #51 from AndersAbel/39SignValFixed

Cleaned up version of #39 Extended signature validation to support signed assertions