Add day/night cycle

[erlend-sh]

First iteration should target this spec, starting with a mere alt-audio implementation:
https://community.amethyst.rs/t/day-night-cycle/868

[QbieShay]

Hello! Since this was labeled as "good first issue" i gave it a try .. This is where I am so far:

• implemented a day-night system (very basic)
• added support for multiple soundtracks
  the code is here:
  master...QbieShay:daynight

I'm new with both Rust and Amethyst (the lines I wrote are amongst my first 200 or so) and I'm stuck at the point where I need a more controllable dj_system. Unfortunately, I feel like, for now, that's too advanced for me. I figured I'd just leave here the code in case someone can find it useful :)