created Creature class

a generic type with a glyph, color, world and pair of coordinates

and a moveBy(mx, my) method
set up creatures and world

CreatureFactory creates a Creature and CreatureAi, links them to one
another, and places the Creature on an empty tile in the World

added method
updated playscreen to handle Player Creature

draw player glyph on appropriate tile
scroll screen position to stay centered on player