Added gems and many other changes. Collision when placing gems is currently bugged.
Gems are working better now. Done some other polishing work.
Improved gems and a whole bunch of other minor changes.

* Increased reverse speed of player.
* Removed ScoreTracker. Score now increased by collecting gems, and is now held with the Game world.
* Missile now leaves a tail after exploding.
* Removed unused BombEnemy class.
* A number of other changes which I can't recall off the top of my head.