Add `fogScale` and `skyColor` options.

Fog looks nicer when it matches the sky, so here's a PR to make them the same color by default :) Have also added `fogScale` and `skyColor` options for tweaking its range/colour.

Before:
![Before](https://f.cloud.github.com/assets/569817/131102/f7ac6982-7048-11e2-9302-4d032bd895be.png)
After:
![After](https://f.cloud.github.com/assets/569817/131101/f7abcedc-7048-11e2-9abb-95ecdd1d949a.png)