Added a visual studio project for using steamworks in a Win32 environment

Added a visual studio project for using steamworks in a Win32 environment, along with instructions on how to support other platforms in a similar way. The visual studio project _links_ to the files where they already were, so there are no changes to the structure of the project.

There are only two changed files:
1. StructPack.cs has some new conditional build switches in. These are pretty much duplicating the existing ones without mentioning "unity" for non unity things.
2. CallbackDispatcher.cs tries to use UnityEngine to log an error. I've again added a build switch in here so if you're building any unity version, you use UnityEngine to log an error, otherwise you use Console.Error.Writeline(...)