Added labels to pie charts

I added labeling functionality to the pie charts. Just add a "label" attribute to the data object array for your pie chart. 

I also added labelFontFamily, labelFontSize, labelFontStyle and labelFontColor attributes to the pie chart defaults object and used them to inform how the labels are drawn.

Not entirely sure how to generate the .min.js file, but if you let me know what to do I can take care of that before merging.

Awesome charting package, submitting to the competition!