add multi fields mapping

http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/mapping-core-types.html#_multi_fields_3
add global aggregation

Conflicts:
	src/main/scala/com/sksamuel/elastic4s/aggregations.scala
add nested aggregation

Conflicts:
	src/main/scala/com/sksamuel/elastic4s/aggregations.scala