Add support for CSS colors, HSL and HWB, PDFColor class, update manual

This is my attempt to overhaul the color handling internals in Prawn. It lets you use nearly all the CSS syntax to represent colors. It also introduces a Prawn::PDFColor base class to store instances of specific colors; of which a subclass Prawn::CSSColor contains all the CSS logic.

I also updated the manual to include a new section on colors in the Basic Concepts chapter.

I think this is well documented and tested, though since its not a trivial change more eyes would be appreciated.