Support enabling and disabling services

Besides installing/uninstalling and starting/stopping a service, the user should be able to enable and disable it.

Use case: in my install.js right now I can only install and start my service, but I cannot schedule it for automatic start on every boot, so installation requires manual operation which I'd rather avoid. My target is to be able to run "sudo npm install -g http://my/host/my/package.tgz", and have everything automatically configured for me.

I'm working on this modification:
- New `enable` property in the systemv module that runs `'update-rc.d '+this.label+' defaults'`
- New `disable` property in the systemv module that runs `'update-rc.d -f '+this.label+' remove'`