add basic projectile management

[znschaffer]

ref #11

Added some basic scaffolding for handling projectiles:

• they track their position, velocity, and sprite (rotation)
• Game state holds all projectiles, updating their position every tick, and drawing them to the screen

Right now, projectiles are living in a List, and being appended using some current information from the ship that fired them

Projectile(self.ship.pos, self.ship.direction, self.ship.calc_forward_facing_velocity(1), self.ship.total_sprite_rotation)

This could be simplified to maybe just passing in the ship's reference and extracting this mess inside the __init__.

Projectiles have a cleanup flag on them - flipped to True if the Game detects they've hit a screen boundary. Would extend this check to asteroid collision after those are implemented.

[znschaffer]

Yup all ready to go for now!