Adding first enemy health implementation.

Basic enemy health and OnHit function added to enemy class.
Add sound when enemy is hit.

Play the enemy killed sound when hit. Eventually we want to use a
different sound, but I'm in haul ass development mode.
Different enemy classes added. Enemy factory added.

Using a factory pattern to construct different enemy types by passing in
the enemyType string. Right now, enemies only have different animations
and types.