documenting color scales

- I didn't document ```DiscreteColorScale``` because I couldn't think of a case I'd want to use it for rather than the other color scale constructors 
- my example plot for ```discrete_color_hue``` came out weirdly tall -- that happened with ```judo Gadfly --package``` but I think not when running ```judo`` on just the one file, or generating the plot by any other means. I'm not sure what happened.
- I opened a couple issues that came up while doing this