Added DamageCircle special ability to sniper ships

- Added DamageCircle class
- Added damage to renderable enum and gametype renderables string array
- Added special ability functions SpawnDamager and EndDamageSupport
- Updated Global.h with those functions
- Updated HealingCircle class to take ship death into account and moved
  run functionality to endStep function
- Added mesh loading for damagecircles into Render.cpp
- Made sure the ship dies at 0 hitpoints, even if it's not the player
  character's ship
- Updated TODO.txt... finally
- Added the mesh file for DamageCircles