Create Game class in score.py, and empty tests.py
Initiate Game with optional balls string
Add tests for Game.balls initialisation
Trustingly split balls into frames

We split into a new frame if the other had two balls or was an 'X' (for
strike); we don't check if these frames make any sense at this stage.
Get rid of frame getter
Calculate score via looping method
Fix deprecated way of doing assertions
Add tests for conversion of balls string to array
Add tests that invalid frames are caught
Test open frames, extra balls and spares
Modify to allow slashes for spares

Was necessary for 0/10 spares but is a nice-to-have for any spare
Add tests for slash spares
Add tests for strikes