Implement ReadToken(string) for Saml and Saml2.

Implement SecurityTokenHandler.ReadToken(string) for Saml and Saml2. This enables SecurityTokenHandlerCollection.ReadToken(string) for all of the default handlers, which allows WsFed to retrieve the token and check expiration. See: https://katanaproject.codeplex.com/SourceControl/changeset/dd843d5623d7ef8caa76244c2ab4009b428efab5