Added health display for mobs during combat.

During combat, players will now see a healthbar appear over the head of
hostile mobs when they are damaged. This healthbar will have two display
options - HEARTS and BAR - which can be changed via the /mobhealth
command.

New Permissions:
mcmmo.mobhealthdisplay - Allows viewing of mob health display
mcmmo.commands.mobhealth - Allows access to the /mobhealth command

New Config Options (config.yml):
Mob_Healthbar.Display_Type - the default health display type
Mob_Healthbar.Display_Time - the amount of time to show health display