Add PointType.XY()
Fix arg names and docs of Fpdf.curve() to reflect what it actually does.
API CHANGE: Fix order of arguments for Fpdf.CubicCurve().

This order is the way coordinates are specified in at least these
2D drawing APIs and is therefore common knowledge or a best practice:
- PDF, see http://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/pdf_reference_1-7.pdf (page 226)
- SVG, see http://www.w3.org/TR/SVG/paths.html#PathDataCubicBezierCommands
- HTML5 Canvas, see http://www.w3.org/TR/2dcontext/

Please note, that this is an API incompatible change.
Add Fpdf.Beziergon().