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.

This makes *one* change to Steamworks.NET itself. StructPack.cs has some new conditional build switches in (pretty much duplicating the existing ones without mentioning "unity" for non unity things).

There's a big problem with the way this is done: Everything is duplicated! Obviously this isn't great. I *suspect* that you can now delete "\Plugins\Steamworks.NET" because everything in there has been duplicated into the visual studio project. I haven't done this though because I don't know anything about unity, and it looks like that folder is something to do with unity?