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.
Wow, that is a good resource. They say sasl_client_done() is recommended but don't say why.
I guess for now, since we are only using SASL for authentication, it's ok to use sasl_client_done()