can draw level and tiles
can scroll with mouse
renaming
active tile
can digg
can not digg surface
load level from file, draw nutrients
able to create monsters from digged tiles
rename Level to Map
can zoom with scrollwheel
can keep pressing for fast digging
monsters deambulate
added middleclass for monster
Extract get_keys to util
extract Idle behavior to a idle status in Monster. Add Stateful
monsters give the illusion of continuous movement