Added Graphics.StrokeDash & Graphics.setStrokeDash()

Signed-off-by: Grant Skinner <info@gskinner.com>
Graphics.command now returns null for empty fills & strokes.

Signed-off-by: Grant Skinner <info@gskinner.com>
Minor improvements to Graphics command object docs.

Signed-off-by: Grant Skinner <info@gskinner.com>