SSL support for JRuby

- Implement MiniSSL for JRuby

- Modify `Binder` and `MiniSSL::Context` to to accommodate the fact
that Java SSL demands a java keystore rather than a key/cert pair

- Change the MiniSSL native extension interface to take a
`MiniSSL::Context` rather than a key/cert pair so that each extension
can grab keys off the context as appropriate

We've been running a build of this and it's been solid, but if/when any issues surface or maintenance is needed, @mention me any time and I'd be glad to help.

Let me know if you have any questions at all, and thanks!