Game Checkpoint #3

Changes:

- Falling down now automatically restarts the level (invisible collider)

- Music has been added. 

- Trail effects have been added for ball movement.

- Particles now emit from Pickup collectibles corresponding to what color they are.

- Yellow swirling particles now emit from player balls when dashing.

- Sound effects are added for everything from hitting bumpers to completing a level to dashing to splitting to merging.

- "Broke down" the Blockers levels into three levels of escalating difficulty.

- Orange blocker enemies now blink between colors to signal they are enemies. 

- A new level has been added to teach players how bumpers work before the final Coaster level.

- Added three gradient skyboxes that have two different colors in them. Skyboxes change every four levels.  The first two have one color on the top half and another color on the bottom half. The final skybox is circular: so there is one color in the middle of the screen and another color outside the screen.

- Added "decorations": neon green and blue and red shapes, arrows on the Blocker level, and yellow gates you pass through on the second level.

- You can now pause the game by pressing Esc, and view the controls from there. You can resume or restart the game or view the controls by clicking on one of the buttons. (To be transparent, the buttons are quite tiny though in WebGL... we didn't have time to fix them.)

- The floor of the final coaster level now looks like an actual roller coaster.

- Exit gates now "hold" balls in place when a ball enters them (so one ball can easily stay in place while the other freely moves around.) A player can dash to get a ball out of an exit gate.