Added a Door Prefab to make transitions in levels.

- The door uses the SceneTransitionGUI prefab which is used to fade
  textures on screen. If you use the door Prefab, make sure you have it in
  the scene. You can find it in Standard Assets -> UI ->
  SceneTransitionGUI.

- To set the door Simply put it where you want, then change the Door
  Anchor to the place where you want the player to appear when using the
  door.