Comment 14 for bug 874241

Meanwhile, I managed a simple workaround (Thank you @5oak for pointing us to a source) that I posted here: http://askubuntu.com/questions/119843/how-to-force-multiple-monitors-correct-resolutions-for-lightdm
It works great for me without issues.
All you have to do is change your laptop screen and monitor screen resolutions in the script as well as the screens' labels.
Use the command: xrand -q to identify them accurately.

http://ubuntuone.com/1AJIiERgE4Yl7d0ICmWh9e

In my case, I am using a Dell XPS 15 L502X connected to a Wide screen (1920x1080) through mini DisplayPort.