Support enabling and disabling services

[RazZziel]

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'

[RazZziel]

Commited pull request: #20

[sellis]

This also now works on redhat systems using chkconfig in my pull request #25

[sellis]

This issue can now be closed.