pre halloween special

• expose trust anchor when authenticating the certificate (requires x509 >= 0.2)
• information about the active session is exposed via epoch : state -> epoch
• distinguish between supported ciphersuites (type ciphersuite) and
  known ciphersuites (type any_ciphersuite)
• distinguish between supported versions by the stack (type tls_version)
  and readable versions (tls_any_version), which might occur in a tls
  record or client_hello read from the network
• support > TLS-1.2 client hellos (as reported by ssllabs.com)
• support iOS 6 devices (who propose NULL ciphers - reported in #160)
• send minimal protocol version in record layer of client hello
  (maximum version is in the client hello itself) (RFC5246, E.1)