distwin

This is the promised distribution target to generate a package including both 32- and 64-bit DLLs for Windows. It works and produces a nice zip, but I am not completely happy with it:

    * It is hard coded to use Visual Studio 2012.
    * You need a zip.exe in your path, which is not so common on Windows. I thought about removing this dependency. There are other obscure options like using PowerShell to generate the script, but everything would require to include another script in libdiscid and I wanted to avoid that.
    * It includes the source in the zip, but only as a copy of the source repository. I am not sure if we should do some initial cmake run here.

@JohnnyJD: You have so far used MinGW, right? I wonder if we shouldn't just stick with that for the 64bit builds, too, instead of using the proprietary MS tools. Have you ever looked into what would be required to create both DLLs in MinGW? Or to cross-compile from Linux?