Add health ability
Add tower 4
Add Thick Sludge unit
Add Shoot ability
Add Look ability
Fix bug where unit health is exactly 0

If health hits exactly 0, then health was being set to max_health again
Add Archer unit
Rename add abilities function in towers
Add tower 5
Add Rescue ability
Resolve dependency issue with captive/rescue

UnitBase imports Rescue, which imports Captive, which imports UnitBase,
which imports Rescue...