Added DamageUpgrade class
Added DamageUpgrade class

brandonnchoii/VOOGASalad,ninasun/game_engine,Jgilhuly/VoogaSalad,PinPinx/TD_Game_Engine,jeremydanielfox/game_engine,keikumata/CS308_Tower_Defense,brandonnchoii/VOOGASalad,ninasun/game_engine,Jgilhuly/VoogaSalad,jeremydanielfox/game_engine,PinPinx/TD_Game_Engine,keikumata/CS308_Tower_Defense