add support for search_type argument
add support for search_type argument

reference:
http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-request-search-type.html

which is useful when:
http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-aggregations.html#_returning_only_aggregation_results

may also add AggregationOnly() for more convenience
snikch/elastigo,allgeek/elastigo,snikch/elastigo,sammi/elastigo,lytics/elastigo,slavau/elastigo,chzyer-dev/elastigo,mikeyoon/elastigo,acsellers/elastigo,otiai10/elastigo,acsellers/elastigo,splicers/elastigo,mattbaird/elastigo,jiangjin/elastigo,snikch/elastigo,allgeek/elastigo,barnslig/elastigo,triptec/elastigo,triptec/elastigo,ramin/elastigo,slavau/elastigo,zaphod-concur/elastigo,weberr13/elastigo,origami/elastigo,dimroc/elastigo,jiangjin/elastigo,acsellers/elastigo,zaphod-concur/elastigo,sammi/elastigo,donnanicolas/elastigo,origami/elastigo,lytics/elastigo,donnanicolas/elastigo,weberr13/elastigo,mattbaird/elastigo,allgeek/elastigo,dimroc/elastigo,splicers/elastigo,barnslig/elastigo,chzyer-dev/elastigo,mikeyoon/elastigo,mattbaird/elastigo,sammi/elastigo,triptec/elastigo,origami/elastigo,jiangjin/elastigo,barnslig/elastigo,donnanicolas/elastigo,otiai10/elastigo,otiai10/elastigo,lytics/elastigo,dimroc/elastigo,zaphod-concur/elastigo,mikeyoon/elastigo,chzyer-dev/elastigo,ramin/elastigo,weberr13/elastigo,splicers/elastigo,slavau/elastigo,ramin/elastigo