Fix the enemy spinning in search mode

Moved the character flip based on crosshair to update loop of shooter
state. Seems to work, crossing fingers.
Fixed Main Menu song starting over

Just moved the load content to Main Menu screen and deleted two lines of
code to stop the main menu music starting over after looking at the
instructions or the story.
Added Game Over overlay and Win Game overlay

Added overlay that shows if all heroes are dead, exits to main menu.
Added overlay (just placeholder for now) that shows if reached the end
of the last level, exits to main menu.
Small balance tweaks

Flight guy does more damage. Speed girl shoots faster. Health consumable
gives more health.
Added OneUp consumable to game

Brings one hero back to life, or if none are dead heals a random hero.
Its gold
Enemy AI tweaks

When enemies are kamikaze they change colors. Changed a little of the
pathfinding code, seems to help a little. They still stop sometimes for
unknown reason.
Fix for bullets disappearing when heroes switch

Guns stop updating when the hero switches or enemy dies. Fixed this by
making the update and draw methods for bullets public, and then updating
them when necessary in the level file.