Add setting of resolution through Paper class.

Add paper.set_view_box and paper.set_pixel_size, which allows setting
the output view box and resolution.
Add "center" argument to arcs.

Make Pen methods arc_left and arc_right take a "center" argument, to
replace the normal radius argument.
Add an example drawing, "tornado".

Add the tornado example, which shows off arcs around a center point.