Allow to deactivate core installer

Hi, I added an option to disable the core installer, which should solve #148. I didn't see any tests for this class, so i didn't add any. I can do if needs be. 

Let me know if the solution is not the preferred one. 

I refactored a little so I didn't have to do the following check in `onNewCodeEvent`

    if (null !=== $this->deployManagerCore) {
        $this->deployManagerCore->doDeploy();
    }