Add a BoxPlot

Added a BoxPlot class using the d3 plugin. There is still a bit a work to be done, such a cleaning up a few hacks, testing and documentation. I'd also like to ensure smooth animations when filtering, rather than a redraw, but that can probably wait until after this pr unless someone want to volunteer :)

![screen shot 2013-09-13 at 4 40 11 pm](https://f.cloud.github.com/assets/197338/1141449/c3574be8-1cb4-11e3-9f96-4b649e1b7468.png)