Add “Script” as a key replacement option

This branch adds `Script` as a key replacement option.

I was having an issue with a service I wrote and globally bound to the `F13` key (System Preferences → Keyboard → Shortcuts). `F13` would launch the service, but *only after* I clicked the app menu (which holds the `Services` sub-menu)—it seems others have had [the same issue](http://superuser.com/a/553134). Thus, PowerKey would not consistently work on my headless media-server.

Note: I did not update the `etc/screenshots/remappingOptions_small.png` screenshot.