Bulk processing

I really like this Kafka river as it works well, is well written, and is compatible with Kafka and Elasticsearch versions.

But in an other application I am writting, I need to execute Bulk request in an elastic search after sending it to Kafka.

In order to do that, I added an indexation mode, telling the river how you want to managed received data. I implemented an other backend to execute bulk requests.

I also need to index received data as soon as possible ( when I have enough messages, I need to insert data into elastic search without waiting to have no more data to read into Kafka ). I added a configuration parameter for that.

Finally, I updated the readme to document how to use my changes. Default beaviour is not changed.

I think it can be usefull to other people so I decided to share these changes.