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.
makes sense. I'm going to put sasl_done() in the cleanup function (mongo_sasl_conn_free) since it will be called when the Ruby object is gc-ed. This seems to make sense since it would parallel the initialization of the Ruby object, which calls sasl_client_init()