Converting River to the d3.chart format

This pull request has the following objectives -
* Break down the `river` chart into layers
 * axis lines
 * label text
 * angle lines on the sides
 * the stacked chart
* Use these layers to make the final chart

Stuff that I'll try and attempt after a while -
* Tooltip in another layer
* Legends (clickable) in another layer

*the following layers should be reusable across multiple charts*