Adds support for contours (d3.geom.contour) and cardinal spline interpolation.
I've extended cardinal spline interpolation to d3.svg.area, and added closed
basis spline interpolation. The spline tension is now set using a named
method, and closed splines can be generated using basis-closed and
cardinal-closed, respectively.
This commit also includes Jeff's nifty clustered force-directed graph example!
The renaming of attributes is totally not worth the hassle of maintaining an
externs file (or using the awkward `foo["bar"]` syntax). The file size
reduction from the advanced optimizations was negligible, besides!
Michael Bostock
Jeffrey Heer