Add an enemy entity

[alexesmet]

Step 1

• Must be physical (doesn't fall through the wall)
• Must have HP

Step 2

• Add attack animation for player
• Attack should reduce hp of the enemy
• Add knockback (optional)
• UPD Must drop an item (optionally: conserve enemie's velocity)

Step 3

• Enemy walks
• Hurts player

Step 4

• Enemy can "see" the player and start walking to it
• Add attack animation/pattern to enemy