Added WiX project for Mono for Windows installer

Requires installed [WiX v 3.9](http://wixtoolset.org/releases/v3.9/stable) (we could also just check in the WiX binaries, whichever you prefer).

`build.bat` builds the installer. It harvests (i.e. generates a WiX source file) all the files in the directory specified in `MONO_FILES_DIR`.