Add hue, saturation, lightness, mix, and with_hue to Color.

[stevepentland]

Added tests for each, and private helper to_hsl to convert an RGB color into its component HSL values. Done in relation to #59

I'm doing these in smaller batches in case anyone else wants to pick some up. Wouldn't want to have someone work on them and then get scooped.

[stevepentland]

Sorry for the delay. I was down with the flu over the weekend and I've just now had the energy to get back to things.

Oh well glad to have you back and I hope you're feeling better! I saw that on Twitter so figured you still weren't feeling well.

[sunjay]

Everything looks great! I deleted some extra blank lines and rebased from master.

Once the build is complete, I will merge this PR! :tada:

Thanks again for all of your excellent work! :smile:

[sunjay]

Once color mixing lands, we'll be able to do cool things like create gradients!

Here's the source code for this:

//! This example shows how you can use color mixing and pen size to produce a smooth gradient

extern crate turtle;

use turtle::{Color, Turtle};

fn main() {
    let mut turtle = Turtle::new();
    turtle.drawing_mut().set_title("Gradients!");
    turtle.set_pen_size(200.0);

    turtle.pen_up();
    turtle.backward(250.0);
    turtle.pen_down();
    turtle.wait_for_click();

    let red: Color = "red".into();
    let white: Color = "white".into();

    for i in 0..100+1 {
        turtle.set_pen_color(red.mix(white, i));
        turtle.forward(5.0);
    }
}

I'm going to add it to the repo as an example once this is merged. :smile: