Changes to be committed:
	modified:   animats.py
	modified:   simulation.py

animats.py:
1. change fruit_hunger and veggie_hunger to random number
2. use a variable to control the ebergy comsumption
3. after one animat die, reproduce one at random place

simulation.py:
1. place 2 tree at top-centered and buttom-centered
2. change the scree size and background size into bigger one