Added possible onclick-handler to slices

- pie.addClickableSlice(percentageSize, percentageStart, "onclickValue",
"bgColor") will add a Slice with the attribute "onclick="onclickValue""
and a pointer as cursor if hovered
- slice.onclick("onclickValue") adds the onclick-information to the
Slice - pie needs pie.update() for this information to become active on
the page