Convert RewardNode to use Sprite

[samreid]

During #82 I noticed that even on a very speedy machine the sprites in rewardNode do not animate smoothly, and there is code overhead in setting up a CanvasNode, and calling drawImage on the reward items. Perhaps it would be simpler and have smoother performance to rewrite using SpriteNode? RewardNode already internally converts nodes to images, so that part is already done.

@jonathanolson or @pixelzoom you are more familiar with Sprite, is this a good idea?

[pixelzoom]

RewardNode seems like a perfect fit for Sprites.js -- transforming and drawing many instances of a small set of images. It would probably be < 2 hours work for someone who is already familiar with Sprites.

[samreid]

@ariel-phet can you please assign and prioritize?

[pixelzoom]

Re-assigning to @kathy-phet to prioritize, since that's no longer @ariel-phet's responsibility.