Fix compatibility with Microsoft Active Directory Federation Services

[friedkiwi]

Microsoft's SAML implementation does not provide the NameID value within the assertion. This causes gosaml2 to not accept the assertion even though it's technically valid.

gosaml2 did not handle this condition gracefully and prevented gosaml2 from working with AD FS and AzureAD. This patch fixes this and updates the README.md to include these tested platforms.