Implement WinTLS

Implement TLS on Windows via native OS support aka. WinTLS.

The implementation uses Schannel, which - unlike most implementations - requires manually reads/write to the socket and passing the data along to the library.

There are quite some pitfalls when it comes to Schannel. I tried my best to comment these pitfalls.
Right now the code is default `--without`, because I didn't have tested it extensively with different servers or even any Windows except Win7.

Please review.