Make labels optional. Allows for a chart that has neither labels nor a legend

I was recently using d3.chart.pie (thanks for the lib), and had to make some customizations to add a few more options. So I thought I'd make some pull requests back to you with the changes in case they may be useful. 

The first makes is possible to have a chart with neither labels nor a legend (my use case already had a legend element present).
Is there a specific reason why you did  `options.labels === undefined` instead of  `!options.labels`?