Make MaxFragment more clear.
Start adding multiversion support; start version 0.4.0.
Add better version support.

- Always check record header for compatible major version but
  ignore minor version when doing a high-level record check.
- Once a version is set for a session, abort if a record doesn't match.
- If renegotiating, abort if a version change is detected.
- As client, always use ServerHello minor version, if compatible.
- As server, when resuming a session, require the same version
  used in the previous session, otherwise, do a full handshake.
Support TLS 1.1.
Release 0.4.0.
Start 0.4.1.