SpriteFrameCache: add the order option to the where method
This way you can request sprites from the Sprite Sheet
in an order that is not necessarily sequential and can show a single
frame several times in the animation.

Example:

    frames = SpriteFrameCache.frames.where prefix: "bear", suffix: ".png", order: [1, 5, 3, 5, 3, 1]
Merge pull request #41 from dabit/sprite-frame-cache-order

Adds:
=> SpriteFrameCache: custom order option for :where