Add iOS7 ScreenSaver.

First version of this cask.

In the future, I would like to be able to start directly `Install iOS 7 screensaver.app` to really install the software (maybe via `after_install`?) instead of linking it. I also would like to add uninstall way. It's just to start "Uninstall.app" in the cask directory.

Any tips (because I don't Ruby) are welcomed!

Thanks.