Add percentiles to Facet Module

// single percentile over complete doc set
json.facet={ median : 'percentile(my_field,50)' }

// calculate multiple percentiles of a function
json.facet={multi:'percentile( mul(f1,f2), 1,10,50,90,99)' }

// calculate the median of each facet bucket and sort by that value
json.facet={categories:{terms:{
  field:cat ,
  sort:'median desc' ,
  facet : {
    median : 'percentile(myfield,50)' ,
    otherstat : 'sum(something_else)'
  }
}}}