Added option to specify position of checkboxes (for highcharts 4)

This takes @trevorwhitney's work from #1847, cleans some stuff up, and updates it to work on highcharts v4.

To clarify what this change does, here is the current checkbox layout (and still the default with this patch or with `plotOptions.series.checkboxPosition: 'right'`:

![checkboxes_right](https://cloud.githubusercontent.com/assets/12112/3124802/47acb864-e781-11e3-8d04-752e256197f5.png)

With this patch, you can now pass `plotOptions.series.checkboxPosition: 'left'` to get this layout: 

![checkboxes_left](https://cloud.githubusercontent.com/assets/12112/3124803/4976408e-e781-11e3-9de6-5b9e68b0aa2d.png)