Initial version of Setup.exe

This PR creates an initial version of Setup.exe. As described [in the spec](https://github.com/Squirrel/Squirrel.Windows.Next/blob/master/specs/Installer.md), our initial version will be a bit simpler, in that instead of downloading and installing the .NET Framework, we'll just whine at them.

### TODO:

- [x] Detect if the .NET Framework is installed
- [x] Raise a TaskDialog if it's not installed
- [x] Unpack and invoke a dummy resource
There's a slight race condition here; someone else could create the directory for you between these two calls. You can instead just do `if (!CreateDirectory(targetDir, NULL) && GetLastError() != ERROR_ALREADY_EXISTS) { boom }`