Sort by average - not for use on high cardinality fields!!!!

Example query

	DELETE test
	PUT test
	{
	  "mappings": {
		"ticker":{
		  "properties": {
			"asset_type":{
			  "type":"keyword"
			},
			"price":{
			  "type":"float"
			}
		  }
		}
	  }
	}

	POST /test/ticker/_bulk
	{"index": {}}
	{"asset_type": "foo","price":10}
	{"index": {}}
	{"asset_type": "foo","price":20}
	{"index": {}}
	{"asset_type": "bar","price":1}
	{"index": {}}
	{"asset_type": "bar","price":10000}


	POST test/_search
	{
	  "size": 0,
	  "aggs": {
		"name": {
		  "terms": {
			"field": "asset_type",
			"size" : 100,
			"order": {
			  "avg_price": "desc"
			}
		  },
		  "aggs": {
			"avg_price": {
			  "avg": {
				"field": "price"
			  }
			}
		  }
		}
	  }
	}