Single ISS File

* We now have only one ISS file; it can build 32- or 32-and-64-bit installers.
* You use a #define to say which installer you want to build (super-easy!)
* Any version of Windows can build both types of installer.
* Added some stuff (for gVim on Windows) to .gitignore.
* Tweaked .gitignore of exe files.
* Clean up and update README a bit.