Refacto item

[PaulQbFeng]

Changes: Mostly cleaning

• Can set an attribute to a stat object using standard setattr method stat.base_armor = 10 , no need to use stat.add("base_armor, 10)
• Implicit init of object. Still need to call super().__init__() if the item has additional stats and effects
• Items have a champion attribute that is a reference to the champion/holder. Similar naming than for spells.
• _percent stats are now expressed in decimal armor_pen_percent=0.3 instead of 30
• enemy_champion changed to target_champion

[Lysxia]

Just target instead of target_champion, because it could be the dummy or Baron Nashor.