Minor Fixes and Game Difficulty Adjustments

Very minor tweaks based on feedback from my very supportive game testers:

• Adjusted spawn rate to reduce number of spawned vehicles.
• Cyclist's movement becomes more sluggish as level increases to simulate depleting stamina.
• Corrected level number when game starts to '0' instead of '1'.
• Leaving the randomly spinning cars in because it's hilarious.
• Adjusted level increment to go up every 450 points, rather than every 1000 points to ramp up difficulty.
• Added a 'secret' when pressing E key in menu and in game.