AWS-16: Images of products are not shown (check S3 stuff)
- added new ChangePermissions task
- moved $changeToDirectory and $runInBackground with their getters and setters to the AbstractServerTask
- created 2 new protected helper method in AbstractServerTask: _prependWithCd() and _appendRunInBackground()
- cleaned up related classes phpdocs
- added return $this where it was absent in setters to support call chaining