Make sure call next plugin when one is finished

What I did:
Removed the for in loop which called the plugins sequentially. Instead
of that I created a callPlugin function which makes it easier to call
the testJSON method and generate callbacks for it.

Three callbacks has been created:
  getOnSpamCallback Returns a function which will be called when spam
  found

  getOkCallback Returns a function which will be called when spam
  found

  getNextCallback Returns a function which called when the plugin calls
  the next function. It does the important stuff:
    Calls he plugin until it called all. It makes sure that the new
    plugin is called only after the previous plugin finsihed.

I also cleaned up the code a bit (JSLINT)