Volume series is now a more generic bar series

 + Assimilated much of our CSS wrt. up / down days
 + Simplified the output SVG for the volume series

The up / down concept which previously involved hard-coded 'open' and 'close' properties has been replaced with a function that can be used to add a class for each bar. The following code produces the same volume series as before:

```
var volume = fc.series.bar()
  .xScale(x)
  .yScale(y2)
  .classForBar( function(d) {
     return d.close > d.open ? 'up-day' : 'down-day'
   });
```