*
@method watch(<obj>, <prop>, <fn>)
@returns Nothing
@short Watches a property of <obj> and runs <fn> when it changes.
@extra <fn> is passed three arguments: the property <prop>, the old value, and the new value. The return value of [fn] will be set as the new value. This method is useful for things such as validating or cleaning the value when it is set. Warning: this method WILL NOT work in browsers that don't support %Object.defineProperty%. This notably includes IE 8 and below, and Opera. This is the only method in Sugar that is not fully compatible with all browsers. %watch% is available as an instance method on extended objects.
@example

Object.watch({ foo: 'bar' }, 'foo', function(prop, oldVal, newVal) {
// Will be run when the property 'foo' is set on the object.
});
Object.extended().watch({ foo: 'bar' }, 'foo', function(prop, oldVal, newVal) {
// Will be run when the property 'foo' is set on the object.
});

*
function(obj, prop, fn) {
      if(!definePropertySupport) return;
      var value = obj[prop];
      object.defineProperty(obj, prop, {
        'enumerable'  : true,
        'configurable': true,
        'get': function() {
          return value;
        },
        'set': function(to) {
          value = fn.call(obj, prop, value, to);
        }
      });
    }