Minor music change.
Merge https://github.com/kevcoxe/Four-to-the-Fifth into kevin
merge kevin's options ui + general cleanup.
Fix title bg overlapping start button.
Add all of Rhea's image files. New zombie enemies and sprites.
Add ammo to lab map.
Add Rocket sprite into game.
Add all of Rhea's sounds.
Shot delay for enemies reduced to increase difficulty.
The options label should really be called 'controls'.
Add a metal song.
Add a good amount to the lab map.
Add cleaner music track control. Just call play_next_track() now.
Add endgame victory track.
Add credits in readme.
Add credits in readme.
Fix readme credits.
Normalize audio levels.
Change track order.
Updated Gatling

Added in the new gatling gun images