Add collision detection for ship v ship

Hitting an enemy ship destroys it currently and plays
explosion animation
Use debugging to adjust hit bounds of enemy ship

Debugging showed the bounding box for enemies was too large,
so make it smaller.
Add collisions for bullets v enemy ships

Shooting an emeny kills the ship and the bullet and plays and explosion.
Add player health and display it at the top

Use built in text and health/damage function.