Add support for statistical facet in pie chart

Hi, folks,

We had add support for statistical facet in pie chart.
http://www.elasticsearch.org/guide/reference/api/search/facets/statistical-facet/

Our scenario is that: 
1. We have a large volume data-set, and we could not count them one by one, for example, raw pv log
2. We summarized these item-based data by time filed into a new gauge field to reflect hourly behavior, for example hourly-pv by different user type
3. We need show a pie chart on the summary of the new gauge field and some other keys, for example a pie chart to show summarized pv number by different user type

We think these requirement are very common, and so we start the pull request.

Regards,
Mingli