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).
I think if you were to pass in false for options.labels it would then enter the if block and have options.labels be controlled by options.legend. I was trying to preserve the old behavior while allowing options.labels to override the options.legend check completely. If the two were to be completely distinct (with legend not affecting labels), then we would probably not need this.