PLEASE REFER TO MY OTHER ANSWER
DON'T FOLLOW THESE INSTRUCTIONS. THEY ARE NOT NEEDED ANYMORE BECAUSE OF THE RECENT UPDATE OF THE GRAPHICAL STACK THAT OCURRED ON ELEMENTARY FREYA
This is how I managed to install bumblebee and the nvidia driver in elementary OS. Unfortunately, I am unable to install any version higher than nvidia-331. Every attempt to install nvidia official drivers or xorg-edgers drivers or any other driver different from the 331 ended up with no login screen.
I was helped by the fact that I installed elementary OS on a Btrfs partition. I would strongly encourage everyone to do that before trying to mess with the graphics driver. The funny thing is that I have done several installations of the driver in Ubuntu machines before, for some reason it is a lot harder to do it on elementary OS. This is a nice operating system, it definitely deserves a try, but it can be a little rough at the edges at this point. I also plan to update in here for the other modifications I needed to do to the system.
So only follow this instructions if you can afford losing everything on you machine, do it for your own risk, ok?
If you decide to install elementary OS on a BTRFS partition, go and do your first backup using Timemshift. Then learn how to use the app.
Install bumblebee stable ppa
For some reason the bumblebee package on the repos do not work properly, the deamon keeps turning off. Also, the xorg-edgers ppa is incompatible with the modified version of lighdm from elementary OS and, even though, the installation can be done succesfully, you will end up stuck on a black screen in the first reboot after the installation.
The solution I found is to use this old ppa. I know, this is not perfect, but this was the only way to get bumblebee to work in my machine, and maybe this is also your case:
sudo add-apt-repository ppa:bumblebee/stable -y
sudo apt-get update
sudo apt-get install bumblebee nvidia-331primus
Then you have to make sure the bumblebee.conf is properly set:
sudo nano /etc/bumblebee/bumblebee.conf
and add the modify as follows ( source ):
[bumblebee] section: after "Driver=" insert "nvidia"
[nvidia-driver] section: after "KernelDriver=" insert "nvidia-XXX"
[nvidia-driver] section: after "LibraryPath=" insert "/usr/lib/nvidia-XXX, /usr/lib/nvidia-331-updates:/usr/lib32/nvidia-XXX"
[nvidia-driver] section: after "XorgModulePath=" insert "/usr/lib/nvidia-XXX/xorg,/usr/lib/xorg/modules"
Where XXX is the nvidia driver you have installed, in the example: nvidia-331
In my case, a simple
sudo service bumblebeed restart did the job, but you may need to restart.