2DPlatformer

Here are my modifications:

1. A new pickup to lower player gravityScale (You can jump high)

2. A new pickup to trig player invincible (Acid only drops one health point, only in Level1)

3. Both pickups incur time-limit effect which is not available in the original project. So I create a new PickupManager to contorl these things.

Minor changes:

4. Score pickups are basically in the acid, so the layer abount ScorePickups are changed.

5. Instructions, levelnames, and credits are inserted into the Mainmenu. Level names are changed

6. The planet color is changed in the Mainmenu.

Credit by Xiaodong LI