Lightning Gun, random level generation and weapon unlocks!

I've added a bunch of cool new stuff in this release:

• Random level generation including new artwork.
• Weapons unlock progression (you can skip it from the options menu if grinding 600 lifetime crates bums you out).
• Additional controls map — now you can use WASD to move, O to shoot, and P/Space to jump.
• Two new guns:
  • Lightning Gun — it could help you clear out an entire room when you're in a pickle!
  • Mine Launcher — use it to strategically place mines that detonate on impact.
• Improved Katana animation and range.
• Random backgrounds.
  
• ...and a bunch of little fixes.

Known bug: enemies can bet stuck in platforms sometimes.