Increase life mechanism based on score.

All changes have been done inside Game.py.
added extra_life_treshold variable that determines how many points
you need to have to gain an extra life, set to 50.

Added inc_life() method that increases lives by 1, and
extra_life_treshold by 50.

Added test for the score equaling the extra_life_treshold, if true then
inc_life() is called.