Added support for replication=async option when storing index document

It's a very simple change to support Asynchronous Replication option when indexing a document on elasticsearch

-----------From Elastic Search Website---------
Asynchronous Replication
By default, the index operation only returns after all shards within the replication group have indexed the document (sync replication). To enable asynchronous replication, causing the replication process to take place in the background, set the replication parameter to async. When asynchronous replication is used, the index operation will return as soon as the operation succeeds on the primary shard.

Thanks