Feature #36 HSP color space, and color.brightness()

Hey,

based on [feature request #36](https://github.com/gka/chroma.js/issues/36), I implemented the HSP color space as explained in [1].
Additionally, similar to color. luminance(), I implemented color.brightness() that returns the perceived brightness P of HSP as a standalone function.

Missing: Interpolation between two colors in hsp. I wasn't sure about [these](https://github.com/gka/chroma.js/blob/master/src/color.coffee#L212-L235) lines in colors.coffee.

[1] http://alienryderflex.com/hsp.html