change `old_passwords` default to disabled:

* it breaks current rails apps that are using the `mysql2` gem

Error is:
        Mysql2::Error: Connection using old (pre-4.1.1) authentication
        protocol refused (client option 'secure_auth' enabled)

This occurs on 5.5 servers even though `secure_auth` is disabled because
`old_passwords=1` generates the pre-4.1.1 14 char hash rather than the
newer style 31 char hash.
update README with new default and contribution
Merge pull request #95 from phlipper/spovich-old-passwords

Disable `old_passwords` by default