Download and run Platypus: http://sveinbjorn.org/platypus
Enter the following settings:
App Name: Generic
Script Path: {path to "shell_open_url" in this repo}
Double click on the Platypus icon and change it to generic.icns from the repository
Add the argument genericURL (make sure to click apply!)
Click Create
IMPORTANT: Make sure your click "use XML" see screenshot below:
And overwrite the Generic app in this repository (because you don't trust me)