Add import/export for HSV and HSL color spaces.

I thought these might be useful to others.  I was inspired by `ChunkyPNG::Color.from_hex` and `ChunkyPNG::Color.to_hex` to add similar methods for the HSV and HSL systems.  HSV/HSL is nice for end-users to work with because the parameters (hue, saturation, brightness) describe colors in the same way many people think of them.

This PR adds code, documentation, and specs for `ChunkyPNG::Color.from_hsv`, `ChunkyPNG::Color.from_hsl`, `ChunkyPNG::Color.to_hsv`, and `ChunkyPNG::Color.to_hsl`.

HSV methods have been aliased to `#from_hsb` and `#to_hsb` because HSB and HSV are used interchangeably; `value` and `brightness` are synonyms in this context.