Contains the following changes

• There is no longer a limit on bullets.  However, a small delay occurs between bullets if you hold down the space bar.
• I added a new obstacle - an electrode which fires a ray from top to bottom.
• Got rid of the jet noise which sounded too much like white noise and added background music instead.
• Difficulty level increases more quickly.

Known glitch: some of the enemy elements jump a pixel or two relative to the background occasionally.  I haven't been able to figure out why this is happening yet, but am investigating.