added interface CritterGroup and implementing classes CatGroup and
RabbitGroup for collections of cats and rabbits. 
Updated BatsEverywhere to include CritterGroup implementing classes. 
Updated Critter move() method so Critters don't go out of the bounds of
town and turn to a different direction to move instead
Merge branch 'master' into catsrabbits
Critter: made drawWhisker() and draw3Whiskers() private. Added
draw6Whiskers() method 
Cat and Rabbit: in drawNose, replaced 2 invocations of draw3Whiskers()
with a single draw6Whiskers() method