Actual Asteroids

[bbergen]

We need some asteroids that slowly rotate around the screen.

Stick with simply large asteroids for now.
Collision, small asteroids, and explosions will be handled in a separate issue, but should be kept in mind during the design and implementation of this issue.

Ensure there is a function for generating random asteroids.

Random asteroids should:

• Have a manageable range of vertices.
• Random speed/direction of rotation
• Always be a complete polygon (bound finite chain of points)
• Be closed polygons
• Fit roughly in the same bounds regardless of number of vertices
• Should never be self-intersecting
• Wrap the screen
• Have all functionality encapsulated in asteroid.c, with only its API exposed in asteroid.h