Refactored and extended spec.

In particular, adapted the spec to the new use of 1536-bit UniformDH rather
than 4096-bit.
Extend the protocol specification.

Explicitely mention that server's must only reply if the client can prove
knowledge of the shared secret in its first TCP message.
Extend the protocol specification.

Refactor the message header format section and document the structure of
session tickets and how the server should manage them.