color.py - add some more color methods

Adds a few methods to the color module

• to_hsv() -> [h, s, v]
• to_hsl() -> [h, s, l]
• get_complementary() -> Color
• change_hue()
• change_saturation_hsv()
• change_saturation_hsl()
• change_value()
• change_lightness()
• darken() (uses change_sat_hsl)
• brighten() (uses change_sat_hsl)
• __mul__() -> Color
• __add__() -> Color
• __sub__() -> Color