I added the following functionality to the game:

1) I added thrust control to the player's ship. Now when the player presses the up arrow key, the player's ship will move in the direction it is pointing.
2) I added shields to the player's ship. When the player presses the SHIFT key shields are enabled.
3) I added the constraint that prevents the shields and thrust from being used at the same time. When thrust is applied shields are turned off. However, if the player applied thrust and releases it, the ship will have forward momentum and shields can be applied to ram into asteroids and other enemy entities.
4) I added the code to load the respective images when thrust is applied, shields are applied and when both are to engaged.
5) I added the shield level bar, which is displayed in the top right of the screen during game play.
6) I cleaned up some old code from previous commits.
I added more features:

1) I added powerups including shields, firepower, health and 250, 500 and 1000 point rewards.
2) I made the power ups disappear after a fixed length of time.
3) I added a score manager.
4) I improved the HUD by adding fire power indicator, score, and current level.

There are two new classes a Powerup class and the score manager.