Implemented d3.svg.arc.cornerRadius 

See #1131. This implementation uses approach C for the case of large corner radius relative to arc length—i.e. using Bézier curve approximations to both align the corner start points with those of neighbouring slices and also touch the inner and outer radius, at the expense of the corners no longer being circular.

This features adds about 1.3kB to the minified library (comparing the results of `make`).

_NB. I went ahead and implemented this approach, though I'm still open to switching to the other approaches if so desired._