POC for Kerberos on Windows with SSPI

I added 5 new files:

* `kerberos_sspi.*`: these are thin wrappers around windows system calls that help with loading DLLs, they're copied exactly from the [node driver's SSPI implementation](https://github.com/christkv/kerberos/tree/master/lib/win32)
* `sasl_sspi.c`, `sasl_sspi.h`: some glue to make it easier to use the windows system calls from Go. Mostly, its just a C adaptation of [Eric Milkie's implementation for the mongo shell](https://github.com/mongodb/mongo/blob/b823d270b570cf4da5629959ee6bfc7cdfd69413/src/mongo/client/sasl_sspi.cpp)
* `sasl_sspi.go`: exposes the same API as `sasl.go` but uses Windows SSPI instead of SASL.

Still a couple outstanding issues, most notably that 1) I haven't added tests yet (see #12, among other limitations which we can discuss), and 2) only ANSI support for now, no unicode, because for some reason my testing setup is blowing up when I try to use unicode.
I don't think I understand the issue. Close cannot be called after a step unless the saslSession is not going to be touched anymore, and if that's the case its resources should be released or we do have leaks. If there are lurking errors, we should understand them as there's likely a fundamental issue in the way we're using the API.