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.
Typo in _sspi_.. I don't think we need those prefixes, by the way. The symbol namespace in the package is pretty clean, unless I misunderstand some particularity of Windows.