add multi fields mapping

http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/mapping-core-types.html#_multi_fields_3
add global aggregation
add nested aggregation
Merge pull request #232 from vbehar/features/aggs_nested

add nested aggregation
Merge pull request #231 from vbehar/features/aggs_global

add global aggregation
Merge pull request #230 from vbehar/features/mapping_multi_fields

add multi fields mapping