Enemies regain health based on difficulty after killing a player

[otsffs]

Minimum Health Recovery is 20% of Maximum Health

Health Recovery Percentage by Difficulty Level: 0 - 0% 1 - 15% 2 - 30% 3 - 45% 4 - 60% 5 - 100%

(Note: Actual recovery is capped at a minimum of 20% of max health.)

[otsffs]

Closes #326