Use argparse to simplify command line parameter parsing.

This produces very similar help output to the previous code. As far as I can tell, `argparse` is built in to Python, so this shouldn't add any extra build dependencies.

The only change in functionality is the addition of the `--output` parameter (`-o` is still there), which was mostly for consistency with the other parameters.

The code size could be reduced further by removing the "alias" variable section:

    (xL, yU, xU, yL) = options.coord
    pixelWidth = options.width
    startColor = options.color
    colorRange = options.colors
    outputName = options.output
    maxDepth = options.depth

and just using the option.* name everywhere else in the code (ie, use `options.width` instead of `pixelWidth`).

-David