Add Sankey demo

I did a quick demo of an openspending powered Sankey diagram using rCharts.

The demo should be able to pull the Sankey files from a URL, eg using the dir path:
sankeyPlot$setLib('http://timelyportfolio.github.io/rCharts_d3_sankey')

but it failed to work for me for some reason.... so I popped the demo files from @timelyportfolio into the examples dir.

We'd need to check license conditions if it remains...