Add Windows binaries

For issue #21.

I have built the 64-bit Windows DLL, on Linux (using `make` and the build dependencies in https://github.com/tomfitzhenry/scrypt/commit/d12fbb17ae4136e88741dc643b1648ca1e70760a ), and tested it on Windows, and it passes tests.

However, I am unfamiliar with the Windows platform, and so am not confident that this binaries works in all situations. I'm raising this pull request in an attempt to get comments from people who are more confident, until it's in a workable state.

This entire pull request is a cleaned up version of https://github.com/FauxFaux/scrypt/commit/57d134ffa5c6e55264e417b85e0befe818126fe4 .