Made the cursor into an image b/c on Mac it disappears when typing

This also unifies the cursor cross platform.
Improved how the characters and guns are created

Also changed some gun specs,
improved the relationship between Guns and Bullets,
    - Now each bullet has a gun and each gun has a character
and moved bullet updating into its own function.
Just for kicks, also added an option to draw the characters as circles
Centered crosshairs again and made them a but smaller

And now they are always on top of everything else
Added simple enemies and health

The enemies start out green, at a random location, facing a random
direction, move in really big circles, and don’t shoot. When hit, they
loose health and slowly turn red, when health is 0% they stop moving.
Ran these through a prettifier to to keep things consistent