Mutiple entries support
This fixes the ability to add multiple entries:
var set = new WeakReferenceSet();
var item1 = {};
var item2 = {};
var item3 = {};
set.addReference(item1);
set.addReference(item2);
set.addReference(item3);
set.get(item1) // => item1
set.get(item2) // => item2
set.get(item3) // => item3