Use property setters for STEALTH and DIVINE_SHIELD as well
Use a property setter for GameTag.ARMOR as well
Rewrite heal/damage system

 - Now uses GameTag.DAMAGE everywhere with a property getter/setter
 - Card definitions now use self.heal() and self.hit() instead of directly
   damaging or healing targets.
 - Card.heal() and Card.hit() will trigger onHeal() and onDamage() where the
   heal and damage logic will be processed. This lets us know what the source
   of damage is and paves the way to implementing spellpower, Betrayal etc.