Added Layers, Flashing Sprites and Swinging Log

-Added Ennemy layer as requested by Ylla.
-Added FlashSprite script. This makes a sprite flash white for a few
frames when hit. Adds Feedback to what can be damaged and what can't.
- To use, put the prefab as a child of the sprite you want flashing, make
  sure both have the same sprite  and size and simply call the Flash()
  function when needed.
- Some more work on swinging log.