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.
Looking at the implementation: http://git.cyrusimap.org/cyrus-sasl/tree/lib/client.c#n271
I guess its ok to call _init() multiple times, as long as _done() is called as often.

This may however be an implementation detail that can change in the future and not to be relied upon... unsure... The manpage doesn't explicitly cover this.. The norm for libs that need to init/cleanedup is to only call these once :]