Added support to build on windows

As mentioned in issue #34, I'm now able to compile on windows.
Tested with Win 8.1 x64, VS 2010.
Now sharing this in case anyone find it useful.

Be aware that there're minor changes to the libssh source for building, and they're ***not*** tested thoroughly.

Also OpenSSL search path is currently hardcoded, see [here](https://github.com/TooTallNate/node-gyp/wiki/Linking-to-OpenSSL), [here](http://slproweb.com/products/Win32OpenSSL.html) and the ```binding.gyp``` file for detail.

Also trying to fix recent sftp-test.js failures.