New (beta) game - Space Invaders!

Still unfinished, but releasing early to allow feedback from the community.

Limitations:
 * Enemies shown as blocks instead of aliens.
 * No multiplayer support yet.

New in:
 - v0.17.0: Destructable barriers to protect you from enemy shots.
 - v0.18.0: Enemies drop down each level.
 - v0.19.0: Better enemy firing.

Please donate to support further development.
