Elasticsearch bulk codec

This PR implements an Elasticsearch bulk codec, and should hopefully fix [LOGSTASH-1445](https://logstash.jira.com/browse/LOGSTASH-1445) and by association [LOGSTASH-928](https://logstash.jira.com/browse/LOGSTASH-928) & [LOGSTASH-1117](https://logstash.jira.com/browse/LOGSTASH-1117).

For decode, it only supports the ```index``` and ```create``` bulk operations and just picks out the document that follows those commands, so the ```_index```, ```_type```, and ```_id``` fields are lost. All other bulk operations are ignored.

Decode tested by ```nc(1)```'ing some bulk data to something like:

```
input {
  tcp {
    host => localhost
    port => 12345
    codec => elasticsearch
  }
}

output {
  stdout {
    codec => rubydebug
  }
}
```

For encode, the ```index```, ```index_type``` and ```document_id``` options are supported same as the other Elasticsearch-based outputs.

Encode tested with:

```
input {
  tcp {
    host => localhost
    port => 12345
    codec => line
    type => test
  }
}

filter {
  uuid {
    target => uuid
  }
}

output {
  rabbitmq {
    host => localhost
    exchange => elasticsearch
    exchange_type => direct
    key => elasticsearch
    codec => elasticsearch {
      document_id => "%{uuid}"
    }
  }
}
```

An Elasticsearch cluster with a river set up pointing to the same RabbitMQ successfully indexes the queue.

Once this is deemed usable I guess the ```elasticsearch_http``` and ```elasticsearch_river``` outputs are made obselete.