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.
You have to be careful with sasl_dispose(). Today we do not use SASL to negotiate a crypto connection so you can call it right after authenticating or to cleanup a failed authentication.
However, if we ever actually use SASL for its full capabilities (e.g. negotiating a real security transport layer, not only authentication) then you'll have to be careful of only sasl_dispose() the connection once you close the connection or have no longer need for it.

Note that sasl_client_done() (the recommended way) was introduced in 2.1.25, while sasl_done() has been available forever.

See also: https://cyrusimap.org/docs/cyrus-sasl/2.1.25/programming.php
Its a very nice resource