Finally: Official Youtube app playing music in background
2. 2013 Youtube app (new design | newest gapps)
3. HALO app from Google Play (free)
1. Download HALO app from Google Play
2. Open it, add any app to the list you want
3. Click "play" button on upper-right (just turn it on
4. Turn on HALO funcionality from taskbar
5. Open Youtube app
6. Play any video you want
7. Open app which you picked up in step 2 by sliding right your finger from HALO circle.
8. Press home button
Voila! You can even turn off halo now
I'm thinking to make video for it. Should I, or there's no need for?