control of d3.svg.arc sweep direction

Allows control of the sweep direction on paths generated with d3.svg.arc.  This is particularly useful when the path will be used for an `<svg:textPath>`, since the sweep direction controls which side of the path the text will attach to (see http://bl.ocks.org/telic/9160262).

For compatibility with existing uses of d3.svg.arc, I've made the negativeSweep accessor fallback to a `false` value when a data object is not supplied and the accessor has not been overridden.