Implement a sun and moon along with a day/night cycle

[ipetkov]

• Sun rises and sets, moon also appears at night time
• Sky color changes based on time of day (day, rising/setting sun, night time)
• Added spheres to the project

I experimented with using a sky box (or sky sphere, now that those are implemented), but I really like how the sky turned out and I decided not to worry about a sky box.

The benefits of the single sky color is that it is easier to animate rising/setting of sun, rather than switch between two sky maps or try to color them differently. I'm open to suggestions if anyone thinks of something better!

[ipetkov]

Merged the changes in master into this branch. Will commit to master later tonight unless anyone has any objections.

@kevlu94 check out my sky implementation and what you think about it replacing your sky prism

[kevlu94]

I like the new implementation, and great job figuring out texture mapping for spheres! Merge whenever you like.