add stats panel, add number formatter

I've added a statistical facet panel which fits my needs (ie: total and mean for http request's bytes).

I also added a way to format numbers: '15,453,233,976 docs' is more readable than 15453233976 docs :)

The number filter use the angularjs one, it will benefit from the localisation module.
The metric filter is for base 10 (ie: 10M), the binary filter is for base 2 (ie 5 GiB)