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.
Actually, the sasl state is cleaned up. The context is wrapped in a struct here
https://github.com/mongodb/mongo-ruby-driver/pull/459/files#diff-2480dc4faaa354322f078919b8966823R87

and a function, mongo_sasl_conn_free is called when it is garbage collection:
https://github.com/mongodb/mongo-ruby-driver/pull/459/files#diff-2480dc4faaa354322f078919b8966823R5