added 'index' to tooltip:format:value arguments

Add `index` to tooltip:format:value arguments to enhance data processing. 
Example: to convert values in the tooltip from e.g. percentages to absolute values, you have to know the `length` the chosen data set.

```
tooltip: {
  format: {
    value: function (value, ratio, id, index) {

      var items = chartData[7][index + 1];

      if (id == 'progress') {
        value = value + '%';
      } else {
        value = (value * items) / 100;
      }

      return value;
    }
  }
}
````

Fiddle: http://jsfiddle.net/gopeter/Yq3DW/7/

I think issues like #180 can benefit from this change.