draw_rectangle returns a new image [BUG]

[martinkozle]

Describe the bug All of the functions modify the passed image (numpy array). While the draw_rectangle function creates a copy of the passed img called output, adds the rectangles to it and returns that. Therefore the examples given in the README do not in fact draw a rectangle.

Expected behavior Either all functions need to return a new image, or draw_rectangle needs to modify the passed image.

Version 0.1.0