Implemented elliptical arcs for PShapeSVG

Fixes #169. Implemented according to:
http://www.w3.org/TR/SVG/paths.html#PathDataEllipticalArcCommands
http://www.w3.org/TR/SVG/implnote.html#ArcImplementationNotes
http://www.spaceroots.org/documents/ellipse/node22.html

To give it a go, you can run this sketch in tweak mode:
https://gist.github.com/JakubValtar/9d06d99a09646bde4b67