Add the ability to set fill_color, stroke_color, etc. with something
other than a string. This class should have a method color_str that
returns a hex string representing the color.

Apparently, in ruby 1.9.3 the stabby lambda, at least when used as a
when in a case statement, can't have a space before the parenthesis.

Add the new color classes and a factory for them. Modify existing code
to use them.

Add color classes. Modify prawn to use them internally. Make all specs
work.