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 
Let's add a test to ensure that the event coming out of `run` is a `LogStash::Event`