Fix get_final_score() and set_bids()

+ Make get_final_score() a proper function so that the input can be
validated and to get logic out of spades.py and int the module (where
it belongs!)
+ Update set_spades_bids() so that bids can’t total 13
+ Create second set_bids() method since this is a spades-specific rule
that other games may not share