Scoring system

This pull request implements the scoring system, as described in issue #92.

Also, per issue #93, par times were added to each level, although these times have not been tested.

There is a new struct called LevelScoreData that is created when the user completes a level, and it is passed to any method that is added to the Simulation.OnComplete event. An example of that can be seen in the OnLevelComplete() method in GameplayScreen.cs. You can see your score in the console, although that should change after Michael implements a level complete screen.

Finally, a small tweak was made to level 12. And that's it :)