Here's a simple way to get the nexus 5's launcher on our device. Just flash this
file via recovery. Then, select "Launcher" as your default launcher. Improved Google Search (Velvet) is also installed since it is needed for this launcher to work.
To switch back to old launcher just change it from settings.
Note :- I take no credit for this work. I just came across it. It worked without any problems on my device. I am not responsible if you mess something up.
Grab it : http://d-h.st/xcA