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:

- [ ] Detect if the .NET Framework is installed
- [ ] Raise a TaskDialog if it's not installed