Timeline.addTweenAt

[gskinner]

This is unnecessary for the vast majority of tweens, but could be helpful for the rare case where people are trying to add custom content into exported MovieClip elements at runtime.

See:
http://community.createjs.com/discussions/tweenjs/563-timelineaddtween-problems

[gskinner]

The .tweens property is now public, which partly resolves this, though it's still not ideal since we want to ensure tweens are removed from any previous Timeline before being added. I'm going to leave this open for now, and we'll see if it comes up again.