initial screensaver work

First trial of screensaver work.

Manual testing: on a new user, symlinking to ~/Library/Screen Savers works, but only after a logout and login.

However I don't know how to test different versions of brew-cask. For some reason `brew install brew-cask` still installs phinze/homebrew-cask even though I edited the brew-cask.rb formula in my taps...