openssl: add handling for base 0 to new and to_s

In OpenSSL::BN, base 0 is used in `to_s` and `new` to convert to and from a multi precision integer as specified in [RFC #4251 Section 5: mpint](http://tools.ietf.org/html/rfc4251#page-8).
I stumbled over this while fixing a [pull request for sshkey](https://github.com/bensie/sshkey/pull/13) running under JRuby.
That's also what I used for testing - I ran `rake test` for sshkey with the JRuby rake implementation.