Can't enable proprietary nVidia driver on debian 8 with bumblebee

I am running the Debian 8.0 64bit (Jessie, the stable release) on my HP laptop. I am having hard time in installing the proprietary nVidia driver of my graphic card 01:00.0 VGA compatible controller : NVIDIA Corporation GF108M [GeForce GT 630M] [10de:0de9] (rev ff) I have followed step by step the guides on debian wiki. Bumblebee is required since my graphic card comes with nvidia optimus. So far so good, I think I have all the right packages required on my machine. Now I have to enable bumblebee. It turns out the right command is sudo optirun nvidia-settings -c :8 So a graphic front-end appears, I leave all the defaul checks and I have just to save the configuration file in the /etc/X11 folder as xorg.conf. But at the system restart I get the black screen and x can't start, it says that there are "No screens found". Here it is the log

[    74.012] (II) xfree86: Adding drm device (/dev/dri/card0)
[    74.014] (--) PCI:*(0:0:2:0) 8086:0126:103c:181d rev 9, Mem @ 0xd4000000/4194304, 0xc0000000/268435456, I/O @ 0x000050$
[    74.014] (II) LoadModule: "glx"
[    74.015] (II) Loading /usr/lib/xorg/modules/extensions/libglx.so
[    74.017] (II) Module glx: vendor="X.Org Foundation"
[    74.017]    compiled for 1.16.4, module version = 1.0.0
[    74.017]    ABI class: X.Org Server Extension, version 8.0
[    74.017] (==) AIGLX enabled
[    74.017] (II) LoadModule: "nvidia"
[    74.017] (WW) Warning, couldn't open module nvidia
[    74.017] (II) UnloadModule: "nvidia"
[    74.017] (II) Unloading nvidia
[    74.018] (EE) Failed to load module "nvidia" (module does not exist, 0)
[    74.018] (EE) No drivers available.
[    74.018] (EE)
Fatal server error:
[    74.018] (EE) no screens found(EE)
[    74.018] (EE)
Please consult the The X.Org Foundation support
         at ....
 for help.
[    74.018] (EE) Please also check the log file at "/home/zarathushtra/.local/share/xorg/Xorg.0.log" for additional infor$
[    74.018] (EE)