Add PDFEllipse to draw circles and ellipses 

Add a PDFEllipse object to allow drawing ellipses and circles in the PDF document. This is a little tricky because there isn't a ellipse primitive in the PDF standard. Instead this uses four bezier cubic spline curves connected to create the drawing. 

As a consequence I refactored the common code from PDFLine, PDFRectangle, and the new PDFEllipse into a PDFDraw class to reduce the amount of redundant code. 

Add a ellipsetest as well to show off the ellipse drawing.