Add support for drilldown-charts

[AlexRegenbogen]

Not quite sure if I'm 'reporting' in the right spot (still very much a github-newb ;-)), but here goes.

I'd love to have the possibility of adding drilldowns to my graphs, which, as far as I can see, is not possible right now.

Both addCount and addSeries won't accept multi-dimensional arrays as input, so right now I'm stuck with plain pie-charts, where I would like to have something similar to this one: http://jsfiddle.net/gh/get/jquery/1.9.1/highslide-software/highcharts.com/tree/master/samples/highcharts/demo/pie-donut/