Made new callback checker slightly more readable
Changed names in base projectile class

They were still "generic" from when this class had that name.
Moved game_speed into world object

This is so it can be accessed by systems. Will be helpful for audio.
Added a separator to explosion entity class file
Added "entities/effects" directory
Added audio source component system
Throttled collision sound entity spawning
Tweaked arena bounding

Now uses the entity's radius if it has one as the tolerance, so they are
destroyed when they touch the arena wall. Good for projectiles.
Misc. cleanup and tweaks
Updated TODO
Merge remote-tracking branch 'upstream/master'

Pull audio addition
Broken shotgun and messed with rocket and minigun