nvd3: A new implementation of a basic R/HTMLwidgets/Shiny interface to nvd3.js.

This has been done before, see https://github.com/ramnathv/rNVD3 and
https://github.com/ramnathv/rCharts. However, I had a lot of trouble getting
those to work well for my specific examples, and neither package seems to be
maintained. I seriously considered simply trying to update those packages, but
decided that I wanted to shoot for a vastly simpler and somewhat lower-level
interface between the R and JavaScript code bases.

In particular, there is really only one generic HTML widget in this package.
Each plot type is implemented as a low-level JavaScript program evaluated in
the widget container and generated by the R package as needed. This approach
simply moves some of the JavaScript code in the package into the R code.  That
has the effect of making it easier to experiment with, modify, and create new
plotting functions without needing to explicitly update the R package.

This is currently an early-stage implementation and I only have two plot
types, a bar plot (nvbarplot) and an area plot (nvareaplot). The structure
of this package makes it really simple however to add new plot types, so
more could follow. But nvd3 is a rather broken and unmaintained plotting
library, so maybe not.