Support opacity in all color types

[acalvino4]

Enhancement request & proposal:

The color interface gets an opacity value that can be null. This value is retained through any color type conversions. If not part of an initial color it is simply null. I'm not tied to this implementation, but my point is that I don't think it would be that complicated (feel free to correct me though....). This would also mirror the way that the css specification now allows an optional opacity parameter for both hsl() and rgb() functions, rather than considering them to be different color types. This proposal would also mean that the opacity information is never lost if it was ever there (if someone wanted to explicitly make it 0, they could do so via an adjustment function of some sort).