Create Mac OS X boot disk image
Go into the downloaded installer app, e.g.
cd /Applications/Install\ OS\ X\ Yosemite.app/
and go into the Resources/MacOS folder, in the Terminal.
cd Resources/MacOS/
look for an executable called "increateinstallmedia".
If you can't find, it, try
find ./ -name "createinstallmedia" -print
Once you find it, run this:
./createinstallmedia --volume /path/to/target --applicationpath /Applications/Install\ OS\ X\ Yosemite.app/ --nointeraction