Allow to deactivate core installer

[AydinHassan]

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();
}

[Flyingmana]

I dont like the refactoring part, because it would make merging with the 3.x branch harder, but as I did change a lot in this area anyway, I could live with it.

A little addition I would like to see, add a var dockblock to the changed class property ( @ var DeployManager[] ) to show its an array of this objects

[AydinHassan]

@Flyingmana done :)

[AydinHassan]

Thanks!