Enhancement: Allow to format pie chart labels (e.g percentage with decimals)

By default PieChart uses `d3.format('%')` on percentage labels and it doesn't allow to customize it.
The only customization allowed on this matter is `valueFormat` which only affect the tooltip value, see [/src/models/pieChart.js#L38](https://github.com/novus/nvd3/blob/master/src/models/pieChart.js#L38)

This PR adds the `labelFormat` option in order to allow formatting chart label, this is particularly useful in scenarios when you want to show the fractional part (e.g percentages with some decimals)