Add Tween and Timeline mixins

[lettertwo]

These are React component mixins that add createTween and createTimeline functions.

They should:

• accept the same args that new Tween or new Timeline would
• return the created Tween or Timeline (for chaining)
• automatically kill() them when the target component unmounts

[matthewwithanm]

Should they accept a target or should it be assumed to be the component instance?

[lettertwo]

hmm, not sure. using the chaining methods (like from, to, and fromTo) becomes an issue then—we'd have to implement our own wrapper with those methods that omit the target argument, i guess.

[matthewwithanm]

Maybe we just expose partially applied forms of those (tweenTo, tweenFrom, and tweenFromTo) in addition to the create*s? Cut down on some of the boilerplate.