Feature/allow the use of metadata

- Allow to store Elasticsearch document information in the `@metadata` field or somewhere else.
- Allow the user to specify which fields to move,  the default is _index, _type, _id

I took this https://github.com/elasticsearch/logstash/pull/1644  as the original starting point 
Confirmed behavior change - 

* Logstash 1.4.2 used `_source` as the Event content: https://github.com/elasticsearch/logstash/blob/v1.4.2/lib/logstash/inputs/elasticsearch.rb#L107-L115
* This new version doesn't. We should revert - the `_source` is the event, and the docinfo/metadata is the other stuff (`_id`, `_index`, `_type`, etc)