Portable version of todotxt

Hi Ben!

OK - it wasn't really possible to modify the Inno Installer, so it could be used both by admin and non-admin users.

Instead I decided to work on a portable version of todotxt, and I guess that should solve both #160 and #193.

For compiling todotxt, and creating both the installer and the portable version, it should now be enough to simply execute `Create-Installer.bat` in the `Installer` directory. The assets should appear in the `Output` directory.

Let me know if I missed something, or something doesn't appear to be working correctly :)

/ Regin