Prevent animated values from leaving expected bounds

[ryanmoelter]

Resolves #4. Sometimes, a value will leave the expected bounds because of chained or delayed animations, and this can cause problems in some cases. But not anymore.

• Clarify committed vs. started animations
  • Committed animations have been scheduled and the set containing them has been .start()ed.
  • Started animations are currently running.
• Cancel committed animations if another animation on the same subject and property is started before they start.
• Prevent animations declared in the same set from cancelling each other by adding a concept of "fully committed" animations.

For an animation that does this if you trigger it once:

It used to fly off the screen like so if you triggered it multiple times:

But now it stays neatly within the expected bounds of the individually-queued animations:

[narenmanoharan]

LGTM. Other than the minor comment.