BIG UPDATE! - Improving the Experience

I made some changes which although sound small, really improve the overall experience in the game. These include:

- Making turret icons red & un-interactable when they can't be purchased

- Changing around the main menu organization

- Using PlayerPrefs to save Audio Settings for your re-entry after closing the game

- Your username now shows up for the entire play session

- Added a new turret, well erhm, an Animal-Themed Turret! (Hint: Half Life 2)

- And many more tweaks and bugfixes