Split enabling of Screen Sharing service from exposing the Screen Sharing.app

- screen_sharing_app => copies 'Screen Sharing.app' to /Applications, if present
  - sNEW: kip check if 'Screen Sharing.app' is not present
- screen_sharing => enables screen-sharing service for all user
Merge pull request #6 from pivotal-sprout/ap/screen_sharing_cleanup

Split enabling of Screen Sharing service from exposing the app