Area change and Centipede color

- Added "areas" to the game controller that signify when color changes.
areas change every 5 rounds, but if we want every wave just replace it
with wave numbers. change the area number in GameController to test it
- Centipedes changes color every new area, with 5 different colors to
change with.
- Changed the centipede head prefab to not be hot pin (sorry I was
screwing with colors earlier)
- a slightly bigger single centipede will spawn at the last wave of each
area.