Optimization pass + bug fixes

* Gameplay should be smoother across the board, with almost no FPS
drops. Where FPS drops do occur, they shouldn't be ruining gameplay.
- Fixed requestAnimationFrame usage. Game object drawing should no
longer occur twice as fast each time a new mission is started.
- Replaced all image elements with a background-image css property in
the parent div.
- Adjusted hp bars
- Moved the width/height properties to the GameObject class
- Fixed a bug where a storm cloud would appear on screen after a mission
had ended
- Added memory functionality to the supplier. Each supplier should now
know which fighters it has supplied, preventing a constant jQuery
background-image property accessing for each fighter within range.