Added second boss type, the Zombat. This zombie has the strange idea that it can fly, and flaps its arms rapidly to float across the ground. Despite looking ridiculous, they are very deadly. They are fast, and if they get close to the player, they can drain his health to replenish their own. They have a large amount of health, and appear in groups of 5 during a boss wave.

Corrected issue where power-up messages were displaying their duration in milliseconds.
Merge branch 'master' of https://github.com/packetpirate/Generic-Zombie-Shooter.git