Add APIs for sprite animation speeds
Add APIs for sprite animation speeds

New in this commit:

- Fiesta.Sprite.getAnimateSpeed
- Fiesta.Sprite.setAnimateSpeed
- Fiesta.Sprite.stopAnimation
- Inside of Fiesta.Sprite.animate, it will no longer animate if the
  animation speed is 0 (or less)

EvanHahn/Fiesta.js