RUBY-530 sasl gssapi support for MRI

We currently support kerberos authentication for JRuby.
This pull request adds a c extension that allows you to use Kerberos when on MRI.
hmm, ok. I've been able to create multiple authenticator objects (and thus call (a_init) / (sasl_client_init()) more than once per process when testing. Maybe it has only worked because the sasl context is identical between tests ? In other words, I'm always authenticating to the same instance with the same credentials.

I need to find out about a cleanup hook for a Ruby C extension. I'd imagine there's a function that is called when garbage collecting the Ruby object from which I can call sasl_done()... thanks for pointing that out.