Do not conflict with gnome-screensaver


This change makes the following changes:

1. Remove conflicting files with gnome-screensaver.
  
  If you accept my another pull request (#11), cinnamon-screensaver's D-Bus API will remain fully compatible with gnome-screensaver. In this case, there is no need to replace the original 'gnome-screensaver-command', because it can activate and use cinnamon-screensaver also.

2. Autostart only in Cinnamon sessions.
  
  GNOME fallback mode, GNOME Shell (if runs without GDM) and Unity use gnome-screensaver. To avoid race condition, cinnamon-screensaver should not be autostarted in these sessions, only in Cinnamon sessions.