Take damage from enemies

[emurnane]

Just as player gains points for destroying enemy ships (see Issue #18), if player's ship collides with an enemy ship, the player should lose points.

Different types of enemies can inflict different amounts of damage:

• 10 points = slow enemy (5 or more recipients), 20 points = medium speed enemy (2-4 recipients), 30 points = fast enemy (1 recipient, i.e., the player)
• 10 points = small enemy (email length 1-xx characters long), 20 points = medium sized enemy (email length xx-yy characters long), 30 points = large enemy (email length greater than yy characters long)