Random color generation

This PR adds `ol.color.nth`. It looks like this:
![random-colors](https://f.cloud.github.com/assets/6942/1948480/c5336012-8093-11e3-9fcc-d5e5119a9b9c.png)
Basically, colors 0, 1, and 2 are red, green, and blue.
Colors 3, 4, and 5 are between them (i.e. yellow, cyan, and magenta).
The next few colors are between those.
And so on.

Everything happens in HSL space. This gives a reasonable algorithm for generating random colors with different hues. The user can also use a few bits to generate saturation or luminosity. See the example for details.