Addressing #53.

• Adds a radial progress bar around the outside of the count down code.
• Updates game logic so the timer only goes down if both gaze steady and head speed is above a set threshold.
• Adds dynamic difficulty, increasing the game time and the required gaze steadiness as more games are completed.
• Adjustments to flame speed scaling, now using a scaled flame sprite instead of a particle emitter for flames.
• Added a particle emitter for some smoke effects.

Other Things to look at:

• Do we want a scrolling background at launch? see https://github.com/UCL/AstroBalance/tree/st/launch-background-move
• Adaptive difficulty needs refining and discussing with David.
• Head speed measurement implementation will need further refinement to enable Data reporting #20, which may also impact adaptive difficulty.
• Needs better graphics, I've used the background from David, but they are space based, rather than a launch scene.