Minor fixes to AttrPlugin and TimelineLite/Max, plus feature addition to Draggable

- Fixed issue that caused relative values not to render correctly at the end of an "attr" tween (AttrPlugin)

- Fixed issue that could cause child tweens/callbacks not to be found in a TimelineLite/Max after it completes.

- Added enabled() method to Draggable that can be used as a getter/setter.