Added camera support and extracted levels to JSON files (requires Node)

- Levels can now be defined with JSON files
  - This required using a web server (Can't load local JSON file for
    security reasons)
  - Used a simple Node.js Express server for this purpose
- Added camera support
  - Can define camera bounds in level JSON
  - Camera tries to put player near the upper middle of the window
- Some physics tweaks to make gameplay a bit easier