Updated level class, hero class, scene_ingame, and added hero health bar

I added a shared health bar over each hero's head. The enemies now deal
damage. Win condition if all enemies are destroyed. Lose condition if
hero health reaches 0. There is now a currentLevel variable created with
a level.create function. You can change these variables such as the
number of enemies that will spawn ( -1 means infinite enemies), how much
time between each spawn etc. Some of the variables don't do anything yet
and may be removed later.