Update todo list.
Lots of minor fixes and planning.

- Added some todo items.

- Closed the ends of looping thick paths with a false join. Still
  doesn't work for real yet because there's a hairline at the join.

- Made the test running script output .svg files for all the examples.

- Added .svg files to the gitignore.

- Added Pen.path_circle, which is a circle made with path arcs. This can
  be outlined unlike an SVG circle element.

- Removed arc_bug example because it's now a fixed bug with a unittest.
Implement better looping paths.
Update some example code.