Chain Lightning, Refactoring and Dev Keys

More backend stuff in this one but a few things of note!

• Backend refactoring for new Bullet class. 
• Chain lightning overhaul complete: now spawns a lightning field every fourth jump if there's room. May cause lag at higher levels - testing in progress.
• Dev key: F4 will now increase the difficulty and spawn a lot of enemies quickly - great for testing out Chain Lightning's overhaul and lag.

Edit: Light Blade wasn't working at level 1 making the game unplayable. This has been fixed. Sorry to those who tried this and were affected!