Leaderboards! And! Loading Screens!

Two small changes that are a long time coming.

• Implemented Steam Leaderboards for Levels 1 and 2. Obviously, Level 2's leaderboard is hidden right now. Steam rate limits leaderboard score uploads so your score will only upload once you leave the level, and should only keep your highest.
• Added a Load Screen! Finally! This was the oldest requested change for the game, except for deleting all traces of it.

Thanks everyone!