Added ability to have gpt events on slots

Added in curly brases for jslint to stop complaining, and for
performance reasons.

Slot events:
In order to determine if an event on a specific ad has fired, I have
added events per slot. Currently this is done by using the set/get
methods avaliable for the slot object, as I have not yet figured out
how to get and extend the prototype for a slot from the googletag
object.

Example:

var adslot = googletag.defineSlot("/something", [728,90], "ad-123");
adslot.set('events', {'gpt-slot_rendered': function(){
  console.log('ad loaded!');
}});

or for multiple events:

adslot.set('events', {'gpt-slot_rendered': [function(){
     console.log('Ive loaded');
}]});

(though you should really use get('events') and merge in the array of
fns for this to work well)