Space Swarm! prototype update

Change Log:

1. updated sprites for beam shield and asteroid chunk.

2. added new threats: tentacle invader and black cube (aka archon aka monolith).

3. fixed a sloppy piece of code where ALL moving entities modified their local x axis movement by a small random amount.

NOTE: the game's current state does not represent its intended final release! Enemies are intended to spawn in a more gradual progression than what's shown here.