v0.3.1

The following changes have been made based on feedback shortly after release:

• New, deeper shield sound
• Homing missile travels forward a short distance after launch, before seeking the target
• Scores adjusted based on enemy ship difficulty
• Score bonus on wave completion
• Bugfix: fixed player 2 score not updating properly in couch co-op

As a result of score changes, the leaderboard has been reset.