Beta upgrades: enemy weapons, debris

Enemy Prey-A ships now have turrets and fire at the player.

Ships, turrets, and munitions now use a more unified architecture to
avoid duplicate code.

Ship class now has animation lists for firing and reloading.

Ship now fires at the _end_ of the fire time. With plasma bolt turrets,
this is accompanied by a charge animation.

Some debris particles added to ship destruction sequence. This includes
24 sprites built in Maya. Debris particles are lit via normal map, and
use a fade-out that makes them disappear into the background.

Particle now has a mixColorAndAlpha method. The old method of
color(sourceColor, alpha) was just horribly prone to losing everything
down an alpha channel somewhere. This one makes the channels explicit
and lets us have nice fading normal-mapped particle systems.

Numerous tweaks to particle behaviours.