Documenting changes from 1.2 to 1.3

[hannestschofenig]

Based on discussion on the TLS mailing list and feedback from JohnM here is a list of items that changed:

Rekeying with Ephemeral Diffie-Hellman in a connection (this was used by DTLS/SCTP).
Post-Handshake server authentication (this was used by DTLS/SCTP).
Rekeying of the exporter secret (this was used by DTLS/SCTP).
Flight #4 (this was used by EAP-TLS 1.2).
Combining certificates with PSK (this was used by 3GPP)
Providing a PSK hint (this was used by 3GPP)

[hannestschofenig]

#48

[thomas-fossati]

Done in #48