revise backwards compatibility & fix ClientHello version

This does the following:
1) Reorganize the backwards compatibility section into subsections for easier readability
2) Freezes the ClientHello record layer version to TLS 1.0 (its effective compatible version)
3) Adds explicit requirement that all other record layer versions match the negotiated version

This plus PR #105 should be sufficient to close issue #54.

The record layer requirements were proposed by Brian Smith on the list.
http://www.ietf.org/mail-archive/web/tls/current/msg14870.html
(also mentioned by him in other discussions)