Support for value_count aggregation using script

[sn00011]

For now, there is only option 'field' available, but not 'script':

Counting the values generated by a script:

{
...,

"aggs" : {
    "grades_count" : { "value_count" : { "script" : "doc['grade'].value" } }
}

}

http://www.elastic.co/guide/en/elasticsearch/reference/1.4/search-aggregations-metrics-valuecount-aggregation.html#_script_7

[sn00011]

Currently i am using something like this get around this issue:

type ScriptAggregate struct {
    Script string `json:"script"`
}

func (d *AggregateDsl) ValueCountByScript(script string) *AggregateDsl {
    d.Type = ScriptAggregate{Script: script}
    d.TypeName = "value_count"
    return d
}