[WIP] RUBY-530 sasl gssapi support for MRI

The C code is a disaster right now but I'm putting this up so that I can recruit some people to look at it. I'm aiming to keep the the API of the authenticator class identical to that of the authenticator implemented in java. This would allow us to eventually just have one file like sasl_java.rb that calls methods on the authenticator without having to know whether it's using the java or c extension.