Make enemies chase after the player.

Added update logic for the enemy's IDLE and ATTACKING states. Also added
a placeholder function for determining whether an enemy can see the player.
Spawn an initial set of enemies for the level
Give enemies the ability to attack

Modified the enemy logic so that when they're within range, they can attack.
Attacks deal damage and the player has a health counter which decreases with
each hit that lands.

The HUD now shows the players remaining health, instead of their x,y
coordinates.

Enemies now have several extra parameters to do with attacking: their range
they can attack from, how long the attack takes and how much damage a
successful attack deals. Enemies also have a health counter, but there's no way
for the player to damage them yet.
Give player the ability to attack

The player now has separate moving and attacking states.

Also added code to cull dead enemies.
Add a dying state for enemies

When their health reaches zero, enemies now go into the DYING state before
being culled. This will give us a chance to play an animation to show that
they're dying.