Adding "color" property to Sprite

If set during instantiation, Sprite will now draw a filled rectangle defined by its rect() of property color. (Because it is using fillStyle, this can also be a linear or radial gradient too.)

Width and height, which now default to null, must also be non-null during an instantiation of Sprite when the property color is also set for anything to appear. (Will fail silently.)

Also added a single unit test for default value of property color (should be null).