A few more arcs instead of a composited circle

The way the donut hole is drawn is not optimal: by compositing the canvas, the shadow is removed under the hole and excanvas doesnt support it and just draws a 'bordercolor' colored circle over the pie (look at the attached images).
![innercircle](https://f.cloud.github.com/assets/1590758/1964337/f5b2d67c-82ab-11e3-94ce-591a41cecfe0.PNG)


I asked in the forum if there is a reason the donut is not drawn the same way the pie is, but I got no answer, so here is a pull-request on the pie plugin to draw a real donut and not a circle over another circle.

here is what we should have, with the shadow and excanvas working properly:
![endresult](https://f.cloud.github.com/assets/1590758/1964330/e1843df8-82ab-11e3-9ea8-73636f4c83de.PNG)