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.
I'm a little unsure how ruby land works... but a_init is only allowed to be called once per process (sasl_client_init()).
Also, there is no cleanup for the sasl routine here either.. sasl_done() should be called during shutdown (if sasl_client_init() was called) to cleanup the cyrus sasl state