Made World 1 easier for new players.
Calibrated world 2 for easy mode.
Balanced world 3 for easy mode.

Skip enemy spawn time if no enemies on the level.

Fixed bug with boss 3 that allowed arm-strikes during enemy spawns.

Made some enemy changes to make them easier for easy mode.
Calibrated World 4 easy mode.
Added enemy_spawner class.

Enemies' spawn-period can decrease now as well as their spawn-number. This
allows us to smooth over many spawn curves instead of having a spike when
the spawn doubles in numbers.
Additional bug fixes on various enemies and level spawning.
Removed the background animation and created a new background.
Improved Credits Screen.
Made the title screen background lighter and changed the menu song.
Fixed some bugs with music player.

When you stop_bg on a song, and then start_bg on that same song without
the fade_out finishing, we need to manually set the song back to full
volume.

Also stopped reward game state from stopping bg if there's no rewards.
Additional level adjustments.
Updated credits.