Game difficulty update!

A fairly big update that balances the difficulty a bit more evenly over time

- The spawn rate and walking speed of enemies is now quadratic instead of linear

- A new 'red' enemy type is added with 5 health, but gives 5 score points

- intructions tab is added on the main menu

- different colour backgrounds have been added

- A spawning sound is added for the early game zombies that spawn, but not in late game (because it would be annoying!)