Rover, particle, and audio fixes

Fixes #25
Particles now change color when you are under the cavelight, but will otherwise not showup as bright colored when you are in the dark. This is being done with a capsule trigger collider tagged as "Cavelight." This same technique can be used wherever we want the dust particles to be lit.

Fixes #8
This was a joint effort with Santi, but I also fixed the particle emittors to be the correct size, as well as adding making sure there is only one audio listener on the prefab.

Fixes #9
The Rover now makes sound as it moves. It does this dynamically so that revving up/down is done through script and not by playing separate audio clips. It still needs work smoothing the rev down, but it gets the idea across.

Along with the audio for the rover, I turned down the cavelight audio so that the noises are not obnoxious. Maybe they are still not quiet enough? Let me know, and I'll try to make it better.