Automatic chart sizing

• Chart margins are now computed at render time rather than with a preset value. Calculation is based on label length and font size to give appropriate padding for labels on both sides of the chart.
• Link length is now computed at render time rather than with a preset value. Calculation is based on the chart width (minus the padding) divided by the number of nodes. This can still be overwritten in the options.
• Under the hood, the javascript bindings have been restructured such that instance properties are now accessed as a property of the widget, rather than being passed around by functions. This makes tracing data flow quite a bit simpler.