Yet another 2D Shooter

This is a mini project for a gaming course.

The following changes were added:

1. A Boss level with a boss health bar. I recycled existing sprites for the boss's appearance and created new pixel art to visualize the health bar.  Asteroids for the boss level to add some challenge: an asteroid is randomly spawned and starts falling down, causing damage on collisionNew code was added to program the level behavior and the health bar animation.

2. Player's health visualization. I added three hearts to visualize the remaining lives. Includes pixel art and new code snippets.

3. New enemy/obstacle: I added a skull that spits fire and blocks the way. Includes pixel art, enemy animation, and projectile animation.

4. New music for the menu.

Minor changes:

1.  New look and feel for the menu;

2. Thrust animation.

Created by: Anton Malarev