Add weapons to the README

Also, rename "health cart" to "food cart"
Fix hitboxes for units.

Hitboxes are now correctly placed over sprites.
Also added XnaHelper.DrawRect(Color, Rect, SpriteBatch) as a useful visual
debugging method.
Add unique particles.

Used currently for hollow particles on black hole.
Particles can rotate around origin
BlackHole increases speed & gravity as it fills.

Adds ability to adjust SpeedFactor to change ParticleEffect behavior at
runtime, and MagnitudeFactor to adjust Gravity strength at runtime.
Implement particle effect spawn multiplier.

ParticleEffect.Spawn may now be called with a multiplier to adjust spawn
density at runtime. Black hole increases spawn density as capacity fills.
Black hole explodes when capacity fills.

When black hole reaches or exceeds its capacity, it briefly dissapears,
then begins exploding, firing particles outward and reversing its gravity.
It then disappears.