Super Crab

I've been using the little-crab scenario for teaching
Greenfoot, and whilst adding the features we we're going
to teach next, I got a bit carried away :D

Main additions:
-Key Control using a and d for crab
-Crab eats worms and adds to score counter
-Worms added at random position one at a time
-Lobsters eat crabs and causes game over
-Control for number of lobsters and speed of
lobsters via difficulty chooser.

UPDATE: 18/10/2010 v0.92
-Simplified difficulty setting code
-Added code to prevent lobsters spawning within a given range of crab
-Worms now hafe a lifespan, and if not eaten will die and respawn elsewhere in the world.
-When worms die the new worm they spawn has a shorter lifespan.
-Once the worms lifespan reaches zero, a gameover is triggered.

UPDATE 19/10/2010: v0.94
-Added different worm lifespans for different difficulties
-Fixed bug where after first worm was eaten, aging effects no longer occured