Compatibility with Microsoft ADFS 2.0

Added a short Read Me text file with steps to follow in order to use
PAC4J against ADFS.
Modified SAML2 response validation. NameID is not needed if at least one
Subject Confirmation is present (even without a NameID). BaseID is also
accepted. If NameID is empty, a dummy NameID is used for SAML2
credentials.
Hi Michael! OK, we can build it at the end of Saml2ResponseValidator.validateSubject(), if no real Name ID is found. And the Saml2Client.buildSaml2Credentials() will just read it from the context.