Add some convenience functions with implicit cairo context

Just to make the code easier to use. So it can look like:

	circle(x, y, radius);
	fill();

rather than:

	cr.arc(x, y, radius, 0, 2 * math.pi);
	cr.fill();

Also, provide a set_color() function that accepts a color name as a
string, (using that classic list of color names from X11 I believe).