Elasticsearch bulk update

All changes are for the ElasticSearch output plugin:
1. Support elastic search bulk updates. 
Bulk updates enables sending multiple messages to be indexed in a single REST API call.
The output plugin buffers messages in memory and sends them over to the elasticsearch server in bulk updates. This should dramatically reduce the load on the agent, on the network and on the server.

2. In addition, I added the ability to specify a different index name other than the default logstash-<date> index name through the 'index_type' option.

3. Finally, I changes how the <date> is computed for the index path. Instead of computing it for every message, compute it only once every minute and cache the result. This too should improve CPU load on the agent.