Multiple ElasticSearch Servers

These modification enable the definition of multiple elasticsearch servers (with the same index). If one of the servers defined dissappear it automatically fails over to the next one defined.

The disabled hosts are re-enabled every 5 minutes (configurable) at which point they enter the node queue and are checked again.

```
2013-10-05 13:16:52 -0400 [debug]: plugin/out_elasticsearch.rb:116:block in write: Sending log to es01
2013-10-05 13:16:52 -0400 [info]: plugin/out_elasticsearch.rb:148:disable: Disabling 'es01' (192.168.1.103:9200) it will be automatically reenabled in 300 seconds
2013-10-05 13:16:52 -0400 [debug]: plugin/out_elasticsearch.rb:116:block in write: Sending log to es02
-------
2013-10-05 13:21:57 -0400 [info]: plugin/out_elasticsearch.rb:154:enable: Enabling 'es01' (192.168.1.103:9200)
2013-10-05 13:21:57 -0400 [debug]: plugin/out_elasticsearch.rb:116:block in write: Sending log to es01
2013-10-05 13:22:52 -0400 [trace]: plugin/in_forward.rb:152:initialize: accepted fluent socket object_id=70214561727300
2013-10-05 13:22:52 -0400 [trace]: plugin/in_forward.rb:193:on_close: closed fluent socket object_id=70214561727300
```